Nokia's new Lumia 635 is in the house, so I'll be evaluating it over the next few weeks to see whether it's a viable replacement for the best-selling Lumia 520/521. Despite some disappointingly low-end specs, it appears to be a typically solid Nokia product, with a nice form factor, a gorgeous screen, and one really compelling internal feature I'm quite curious about.

As a replacement for the Lumia 520 and 521, the Nokia Lumia 635 targets the low-end of the market and as such we shouldn't act surprised at some of the less-than-desirable specs. I'll get to the full rundown in a moment, but the one that does rankle is the inexcusable lack of a hardware camera button. This is the first Windows Phone handset to omit this feature, I believe, and certainly the first Lumia (along with its international sibling, the 630). There is absolutely no justification for this decision, in my opinion.

That said, I intend to use this device as a replacement for my trusty Lumia 520, which means that after the evaluation period is over, I'll use it as a media device. In that sense, the 635 should work just fine: I can add up to 128 GB of microSD storage, which is overkill for even a big music, podcast and audiobook collection.

But most people don't have this luxury. The Lumia 635—which is available on both T-Mobile (the version I received) and, soon, on AT&T Wireless here in the United States—is really targeting the low-end of the market. And that means that, for many customers, this will be their actual phone. So they'll use its standard communications capabilities, productivity and entertainment apps, games, and camera. And I'll be testing the device, accordingly, to see how well it performs in these primary functions.

Microsoft Mobile/Nokia, curiously, didn't send me the normal retail packaging, though I did get everything that normally comes in the box. (This is no loss; T-Mobile's retail boxes are just as lousy as anything AT&T can throw together.) I did find the Cortana-themed lunchbox to be amusing, however.

Inside the box, you get the same set of accessories that accompanied the Lumia 520/521, a strange all-in-one power adapter (where the USB cable is fused to the plug) and then a separate, very short USB cable you can use for charging the device on a PC or whatever.

I like the solid feel of the 635, and I prefer its slightly larger size compared to the Lumia 520/521. This is a 520 pictured here for comparison's sake as I have since gotten rid of my Lumia 521, but the 520 and 521 are roughly the same size, so you get the idea.

Lumia 635 (left) and Lumia 520 (right)

As with the 520/521, you can pry off the high quality plastic back, exposing the removable battery, SIM card/slot and microSD slot. This means you can also theoretically replace the back with a different colored back, something I did do with my Lumia 520 (though it required buying the part internationally). I believe that both T-Mobile and AT&T will offer different colored covers, and assume that this time they are interchangeable between carriers (the 520 and 521 covers were not as the devices were strangely and subtly different).

Spec-wise, the Lumia 635 is mostly disappointing, even when you factor in the fact that it's targeting the affordable end of the market. (On T-Mobile, you can purchase a 635 for $0 down and $7 per month for two years, for example, or buy it outright for just $99.) These include:

CPU. Quad-core 1.2 GHz Snapdragon 400 processor.

RAM. 512 MB, which was the basement last year. I'm thinking almost anyone would be better off with 1 GB and would like to see this needle moved.

Display. The 4.5-inch ClearBlack display is gorgeous, and as I've pointed out many times, there is something about Windows Phone's display scaling that makes even a low-res display—like this 854 x 480 unit—look absolutely fine. Yes, we're in an era of HD and up mobile displays. But don't let this one scare you off. (The device uses software-based Back, Start and Search buttons, so when you factor those out, the effective resolution is the Windows Phone standard 800 x 480.)

Storage. 8 GB internal, up to 128 GB via microSD. Now that you can install almost everything on a microSD card thanks to Windows Phone 8.1, this is absolutely acceptable.

Main camera. The 635 sports a (for Nokia) very low end 5 megapixel camera. There is nothing special here: auto focus, 4x digital zoom, f/2.4 aperture, and so on. But I'll test it. (I haven't yet.) Not a big deal, but the camera does provide just the tiniest of camera bumps on the back.

OS. The Lumia 635 ships with both Windows Phone 8.1 and the Lumia Cyan firmware update.

Bundled apps. In addition to all the standard Windows Phone apps, this T-Mobile reviewer unit came with Nokia App Social, HERE Drive+, HERE Maps, Mix Radio, Camera, Care, Cinemagraph, Creative Studio, and Glam Me, Facebook and Hill Climb Racing (game), FM Radio (because it has the required hardware), and T-Mobile My Account, Name ID, TV, Transfer My Data, and Wi-Fi Calling.

Sensor Core. This is the dark horse in all this. Despite the low-end RAM and camera, which sort of dull the impact of this device, the Lumia 635 does come with Nokia's new Sensor Core technology, which lets fitness apps track your activity using the device's built-in hardware, but without impacting battery life. The Bing Health & Fitness app works with this technology, so I'll compare how it works vs. the Fitbit I normally use.

More soon.