Pay no attention to the version number: Windows Phone 8.1 is a major upgrade for Microsoft's smart phone platform, one that provides improvements virtually everywhere. But 8.1 also ships with some major new features: A beta version of the Cortana digital personal assistant which, for now at least, makes Apple's Sir and Google Now look tame by comparison and a notification center that, frankly, Windows Phone should have had years ago.

I've been using Windows Phone 8.1 on a Nokia Lumia Icon for the past couple of weeks, and it passes the "upgrade test," as I think of it: Going back to the previous OS version is painful, and I miss a few big bucket features in 8.1—especially Action Center, the odd name for Windows Phone's new notification center—but also a number of smaller changes. Suffice to say, I'll be taking advantage of the developer preview program—described near the end of this review—to upgrade all of my existing Windows Phone 8 handsets to 8.1 immediately.

What I'm not seeing is any real indication that Windows Phone is being merged into Windows. Yes, there are absolutely features from Windows 8.1 that now appear in Windows Phone, like settings sync, quiet hours, VPN and Workplace, and much more. But this absolutely isn't a "phone version of Windows." It's still very much Windows Phone as we've known it, with its own quirks and uniquely phone-based features and functionality. The Start screen is oriented for portrait mode and still cannot, for example, be used in landscape mode. It's a phone, not a tablet.

I think that's just fine, by the way. From the first moment that Joe Belfiore introduced Windows Phone on the Mobile World Congress stage in early 2010, I've been taken with the cleanness of this superior platform. And for all the underlying, platform-level changes, from a user experience perspective, Windows Phone has yet to veer from its central mission as the most personal of smart phones. That is not just still true in Windows Phone 8.1, it's been turned up to 11.

As noted, Windows Phone 8.1 is in fact a major, major release. If there wasn't already a Windows 8.1 floating around, I have no doubt Microsoft would have branded this one Windows Phone 9. But platform synchronicity certainly makes sense, because users and potential new customers will immediately understand that both Windows 8.1 and Windows Phone 8.1 are of the same generation and will thus have some common features and the promise of better interoperability. Perhaps the two products will be updated in lock-step from version to version (but not with regards to interim updates) going forward, since the same product group at Microsoft is now responsible for both.

Here's a taste of what's new and different in Windows Phone 8.1.

Lock screen

The lock screen has been evolved with a more attractive and, I think, more usable informational layout, with the clock displayed in a larger clock font and the day and date in a smaller font.

Oddly enough, the lock screen settings are identical to Windows Phone 8, but of course 8.1 supports a new type of lock screen app that can take over the display and provide a unique look and feel. I haven't had the chance to use such an app yet, but the few I've seen demonstrated are attractive, and I suspect these types of apps are going to be a big deal.

Action Center

While cynics will point out that both Android and iOS have supported a notification center for quite some time, whatever: This is absolutely necessary and quite welcome, and it's one of the best new features in Windows Phone 8.1.

Action Center is activated by swiping down from the top of the screen (as with Android and iOS). This works from anywhere in Windows Phone, including, interestingly, the lock screen. There are two parts to Action Center, a row of quick settings tiles and then a list of app notifications.

Action Center is configured from the new Notifications + Actions settings, from which you can determine which quick actions appear at the top, whether Action Center appears over the lock screen, and which apps can display notifications (and how).

And Action Center is awesome. You no longer need to find and then navigate into Settings to do things like enable Airplane Mode or toggle screen rotation. And when you dispense with an app notification, it will reflect on the app's tile. For example, when you remove an Outlook Mobile notification about a new email message—by swiping it to the right—the app's tile changes to no longer display the number of unread email messages. Smart.

As with Android and iOS, you can really customize how each app provides notifications too. On an app-by-app basis, you can choose whether its notifications appear in Action Center, whether it displays a notification banner at the top of the screen, which notification sound it uses (if any), and whether it vibrates.

All-in-all, Action Center looks like it accomplishes exactly what you'd expect. You'll wonder how you ever lived without it.

And on a related note, reminders and reminder sounds have been nicely updated. One of the weirder limitations of reminders in previous Windows Phone versions is that all reminder types—text and instant messages, voicemail, email, and other reminders—had to use the same reminder sound. In 8.1, however, they're finally split out so you can choose different sounds for each type of reminder. (This is configured in Settings, Ringtones + Sounds, not Notifications + Actions.)

Start screen

There are a number of changes related to the Start screen.

First, Microsoft now provides a "show more tiles" option for all Windows Phones with a display with a resolution lower than 1920 x 1080. This enables you to display a much denser set of tiles—5 of the smallest tiles, horizontally—a feature that was previously available only on devices with 1080p screens. (Oddly, this option does not appear on devices with a 1080p display, so you cannot toggle it off and display fewer tiles.) This will transform your existing Windows Phone handset in ways I think most people will find quite appealing. It's the one thing I really miss when I move from a Lumia 1520 or Icon back to my Lumia 1020.

The background and accent color options haven't changed since Windows Phone 8, but there's a new Start Background feature that lets you pick a photo to display under the Start screen. You're no doubt familiar with how this works in Windows 8/RT—the background image appears "behind" and "around" the tiles, which are opaque—but in Windows Phone 8.1, it works quite differently. Here, the background image appears only through those tiles that are transparent. It does not appear "between" the tiles.

This design difference makes sense because the Windows Phone Start screen doesn't have any appreciable dead space, as is possible in Windows. So you wouldn't be able to see the image if it was only available between the tiles. But because many Windows Phone tiles are opaque, and not transparent, you'll need to choose your image—and your tile placement—wisely in order to get the best effect.

Also, there's an interesting parallax effect that occurs when you scroll through the Start screen: The background image scrolls at a different speed than the tiles do. As with the background image capability itself, some will love this feature, while others will find it annoying. But while using a background image is optional, the parallax scrolling effect is not: If you enable the image, you get the scrolling effect.

Frankly, this is one of those features that demos well, and it speaks to the user-centric nature of Windows Phone nicely, but I don't really care for it. This one will be pretty polarizing, which some loving it and some not being so impressed.

Cortana

The most hyped feature of Windows Phone 8.1 isn't even really a feature of Windows Phone 8.1, per se: It's available in beta form and then only in the United States. Everyone else will need to wait, and even then it's not completely clear which markets will get it when.

If you do live outside the United States, you will be disappointed to learn that Cortana works surprisingly well. It's basically a new generation of Bing Search, in the sense that it replaces the Bing Search functionality in Windows Phone 8 with a new version that is more universally integrated with everything in the phone and with voice interaction. And... it works.

That said, I'm still not so sure that I'll ever use Cortana, outside perhaps of the car, as the only thing I find more annoying than talking to a phone is being forced to experience other people talking to their phones. But I'm surprised by the wide range of functionality here, and by how well the voice interaction stuff works. This isn't just searching app launching via voice—every version of Windows Phone has supported that stuff, though few people seem to realize it—but is instead a fully integrated digital assistant.

There's a short wizard that lets you set some preferences and interests, which nicely jump starts its machine learning capabilities so it can get to know you better, and I recommend not skipping that. (Or, you can make changes later via a strangely-named Cortana Notebook.) Once you're up and running, Cortana will appear when you press the Search button on the handset.

Everything about Cortana is smart. If you type a search query, she will assume you don't want audio and will simply provide answers and/or search results via text. If you speak to Cortana, she will speak back to you. The voice interaction capabilities are currently better than either Apple Siri or Google Now—though that can certainly change as those platforms improve—and Cortana lets you interact with apps, including third party apps like Facebook and Twitter, right out of the box.

Everything works as expected. For example, if you say, "what's happening today" (or tomorrow) and Cortana will dictate (and display) today's (or tomorrow's) appointments.

Cortana is such a big topic that I'll be writing more about this in future articles, plus in Windows Phone 8.1 Field Guide. I can say after tooling around with it for a few weeks that I'm not personally sold on this kind of interaction—save perhaps in the car—but I do think that many people will absolutely love it. And of course you can always use normal search if Cortana isn't your thing.

Settings sync

Like Windows 8.1, Windows Phone 8.1 now supports OneDrive-based settings sync functionality. It's a bit light compared to the wellspring of settings sync options we see on Windows, but there is theme, app setting, Internet Explorer and password syncing. And where applicable, these settings will sync between Windows and Windows Phone. So if you're saving web passwords in the desktop Windows version of IE and hit the same site in the Windows Phone version, your password will autofill (and vice versa).

Wordflow keyboard

As a Windows Phone user, I've not been able to really experience the Swype-type virtual keyboards that are currently all the rage on Android. My wife—who, frankly, is kind of a Luddite—swears by this keyboard on her Note 2, and almost certainly would never consider the switch to Windows Phone because of this one thing. But with Windows Phone 8.1, this platform now picks up a Swype-type virtual keyboard. And it works very well.

What's nice about this capability—which Microsoft calls "shape typing"—is that it's not an either-or proposition. It's just enabled, and you can either tap-type the old way, or you can glide your fingers from key to key and use the shape typing functionality. The latter is faster right away, and gets much faster as you get used to it. Many will adapt to this and find a normal virtual keyboard to be tedious and slow. This, too, is fairly profound change for Windows Phone.

Note: This capability is not available on the Lock screen PIN pad, which probably makes sense.

Skype

Windows Phone 8.1 is the first version of Microsoft's smart phone OS to include a bundled and integrated version of Skype. That means that Messaging no longer works for Facebook or Messenger chats, and that you must now use Skype. (This is what happened previously in Windows 8.1 as well.) And of course Skype is now accessible from the Phone dialer so that you can "hand off" a normal phone call to Skype, optionally turning a voice call into a video conference.

Internet Explorer 11

Not surprisingly, Windows Phone 8.1 includes a significantly updated version of Internet Explorer which brings this browser up to speed with the version in mainstream Windows. IE 11 for Windows Phone 8.1 includes PC features InPrivate Browsing and Reading View (shown below), and it now supports syncing Favorites, tabs and history between PC and phone versions of the browser.

There's also an updated address bar, unlimited tabs, and other improvements.

Calendar

The Calendar app has been updated with some long-overdue functionality. Now, instead of a default view pivot set of Day, Agenda and To-Do, you get the more useful Today, Tomorrow, and then subsequent day (Monday, Tuesday, etc.) pivots. A new View button in the app bar lets you switch between Day, Week, Month and Year views, just like a big boy calendar. And those other views are pretty neat, and surprisingly useful on the small screen.

But even the views that existed before have been updated. If you tap on a day in the Month view, for example, it expands to show you which appointments you have on that day. (The other views work similarly.) It's pretty slick and, more to the point, useful.

Maps

Bing Maps gets a big update in Windows Phone 8.1 and now includes improved navigation, though you will still need a "drive" app (like Nokia Drive+) for turn-by-turn navigation with voice control.

Maps also integrates with Wi-Fi Sense, so you can find nearby trusted Wi-Fi networks easily.

Microsoft Camera

With Nokia really turning things up a notch with its Pro camera app, it is perhaps no surprise that the built-in Camera app in Windows Phone 8.1 now works in a similar (if simpler) manner. It offers three main modes—photo, burst, and video—and five quick launch buttons for camera roll, camera chooser, flash, lenses, and scenes.

I had sort of hoped and expected that Nokia's Pro camera app would become the default, but this new app is a big step up from the previous version and is a better fit for the average user. Plus, anyone can download Pro camera and make it the default camera app if they wish.

Photos

The Windows Phone 8.1 Photos hub is finally now extensible, so you may soon be able to access your non-Microsoft, cloud-based photo services from this interface. But even without that integration, Photos has been updated visually and no longer sports the sweeping panorama from before. Instead, you see a new layout that is all business, but gets you to your photos quicker.

Xbox Music, Xbox Video, Podcasts and FM Radio

In all previous Windows Phone versions, Microsoft offered a central Music + Videos hub that provided access to its Zune and then Xbox Music, Video and podcasts functionality, as well as FM radio. In 8.1, that hub is gone, and replaced by separate apps: Xbox Music, Xbox Video, Podcasts, and FM Radio. Each works exactly as you'd expect, though Xbox Music seems to still perform pretty slowly.

Quiet hours

Like Windows 8.1, Windows Phone 8.1 supports a "quiet hours" functionality that lets you configure the phone to not bother you during set hours of the day (typically at night, when you'd be asleep). When configured, Windows Phone will silence calls, text messages and notifications, and you can configure an "inner circle" of close friends who can "breakthrough" quiet hours and reach you at any time. This looks like a really neat feature.

Wi-Fi Sense

Windows Phone 8.1 picks up some nice Wi-Fi functionality from Windows 8, including the ability to sync known Wi-Fi networks and their passwords between whatever Windows PCs, tablets and phones you may own. You can now manage known networks right from the phone, and a new Wi-Fi Sense feature lets you automatically and seamlessly connect to Wi-Fi networks—including those that require a bit of personal information—so you can save on your cellular data usage and bill.

Unexpectedly, there's also a way to share Wi-Fi network settings with your Outlook.com, Facebook and Skype contacts, assume they are also using a Wi-Fi Sense-enabled Windows device (which today means a Windows Phone 8.1 handset). It's like settings sync taken to the next level: Settings sync with friends.

Storage Sense

The Windows Phone 8 Phone Storage setting that debuted in Windows Phone 8 has been updated to a new "sense" setting called Storage Sense. As with Windows 8.1, you can now dive into your storage and see exactly what's taking up the space. In Windows Phone 8, you could see that apps were taking up whatever amount of storage, but you couldn't do anything about it. Now, you can see exactly how much space each app and game is using and uninstall the ones you don't want directly from this interface.

And if you have a microSD card, you can now store (new) apps and/or (new) downloads on the card instead of using up internal storage (in addition to music, videos and podcasts, and photos).

Business features

Windows Phone 8.1 includes a number of new features for businesses, including a few—like enterprise VPN and Workplace—that debuted first in Windows. Also available is enterprise wise—which wipes only your corporate data from a phone, and not all data as with a normal remote wipe—S/MIME email encryption, and various improved MDM (mobile device management) capabilities.

Ease of Access controls

Microsoft is rightfully criticized for not doing as much as Apple does for those with vision or hearing disabilities. But Windows Phone 8.1 does include a few additional ease of access controls over its predecessor. Key among them is Narrator, which will read aloud any text that's on the screen, and new options for zooming in on all web content. You can also change the appearance of captions for videos in IE and apps that use the browser to display content.

Screen projection

Windows Phone 8.1 now supports both wired and wireless screen projection, much like Windows 8.1 does. Wired projection occurs over USB—a feature that was previously available only on specially designed handsets Microsoft and its partners used for public demonstrations—while wireless uses Miracast. (But as you might expect, Miracast is limited to newer devices that support it, including the Nokia Lumia 1520, Icon, and 930.)

You can choose to show a dot on the projector/second screen when you touch the screen, which is great for presenting, and configure the screen orientation as well.

Apps

The Apps view—reachable by swiping to the left from the Start screen—has been updated in some subtle ways. For example, now it displays games, which were previously available only in the Games hub. And it also displays key settings, like Battery Saver, Data Sense and Storage Sense, each of which can now be pinned to the Start screen too. That's helpful, but I wish you could still pin any setting to the Start and/or Apps screen.

Store

The Store app, which provides access to Windows Phone Store, has been significantly upgraded in this release. For starters, it's now all about apps (including games). If you want to find music, videos or podcasts, you need to do so through those apps instead. It's also been updated with a new layout, and categories that match those in the Store app in Windows.

This wasn't testable, but with the universal apps Microsoft introduced at Build, it will soon be possible to purchase an app or game on, say, Windows and then get the same title for free on Windows Phone (or vice versa). This is a nice—and overdue—capability.

Availability

Windows Phone 8.1 is a free update for all Windows Phone 8 handsets. Anyone can get it, and while the general public availability will vary by device and carrier, Microsoft expects this process to happen in force by the summer.

Don't want to wait? Anyone who has signed up for the Windows Phone developer program—including the free version—can get the final version of Windows Phone 8.1 right now, for free. All that's missing are whatever carrier and handset maker updates that may be added in the future, and you can always get those whenever Windows Phone 8.1 heads out publicly over the course of this year. Read Get Windows Phone 8.1 as Soon as Possible for more information.

Final thoughts

As you must know, I'm a huge fan of Windows Phone 8.1 and am quite eager to get going with this update on all of my handsets. I'll spend much of today updating them courtesy of the aforementioned developer program, but then I'll be writing a lot more about Windows Phone 8.1 going forward too. Let me know what you want to hear about the most—I know, I know, Cortana—and I'll get cracking. In the meantime, if you invested in Windows Phone 8 and were wondering about the next version of the OS, it's time to celebrate. Windows Phone 8.1 is a huge update. Windows Phone 8.1 is awesome. And Windows Phone 8.1 is here today.