The iPhone 4S carries over the same Retina display its predecessor, the iPhone 4, used. This is both good and bad. On the good news front, the display is crystal clear, with great viewing angles, and its high resolution (a stunning 960 x 640) combined with a high pixel density (326 pixels per inch) provides the device with one of the highest quality displays on any smart phone. On the flipside, the iPhone 4S display is a somewhat small 3.5-inch size, and compared with Android and Windows Phone handsets with larger displays, it can feel a bit cramped, especially when you're using it with a typical, one-handed grip in portrait mode.
Of course, large screens can be too much of a good thing. The Android-based DROID X I use for work, feature an overly-large 4.3-inch display while some newer phones, like the HTC Titan, have even bigger screens. After using a range of devices, I've found that the sweet spot, physically, is somewhere in the middle. That is, a 3.5-inch screen (iPhone 4S/iPod touch) is too small, while a 4.3-4.7 inch screen (DROID X, HTC Titan) is too big. Hello, Samsung Focus? This is Goldylocks. Your 4-inch screen is just right.
(Your results may vary. Smaller people may actually prefer using a smaller device with a correspondingly smaller screen. People with vision issues may need a device with a larger screen. And so on.)
Comparing the screen on the iPhone 4S and Samsung Focus (which utilizes a lower 800 x 480 resolution on a bigger screen size) is interesting. I'd say they're comparable. There is indeed a crispness to the Retina display, and it's easy to see on the detailed iOS 5 home screen, whereas Windows Phone's simpler Start screen doesn't offer much in the way of high resolution graphics. Pictures taken with the iPhone 4S camera look stunning on the Retina displays, whereas pictures (even decent pictures) taken with the Focus simply look acceptable at best.
The real test, of course, is text. And there are two decent opportunities for 1:1 text rendering comparisons on the iPhone 4S and Focus: Their respective web browsers and the Amazon Kindle app.
Note: The following screenshots are meant to represent the relative size and quality of the iPhone 4S and Samsung Focus screens, respectively. However, the quality of the screenshot facilities on each vary, and of course I've had to resize and compress the images to fit on this site. So please understand that these representations aren't perfect.
Viewing the full width of the New York Times web site in portrait mode, the tiny blurb text is unreadable on both but clearer on the iPhone 4S. Zoom in to a column and things get interesting. Text in both is quite clear, though the iPhone renders the text in a bigger font, which is likely an HTML rendering issue. Navigating into an article and zooming to the article column width, the results are similar. The iPhone 4S wins by a hair with a crisper font rendering, but both are quite good.
How about reading an eBook with Kindle? Here, I couldn't notice any difference at all. In both versions of the Kindle app, text was crisp and the background was bright white. (Both were set to manually display at mid-brightness with fonts set to the middle option as well.)
My verdict? The Retina display delivers crisp graphics and text. The screen needs to be a bit bigger: I think a 3.9- or 4-inch display, perhaps even in a device the same size as the iPhone 4S, would be perfect.
By the way, I've been using a Retina display-based iPod touch for over a year, but as anyone who has used both can tell you, that screen isn't quite as good as the one on the iPhone 4/4S, though it's very close. The resolution and size of the display are the same, sure. And from dead center, the displays look identical. But the iPhone 4S screen offers much better viewing angles, whereas the iPod touch display fades quickly as you move off-center. This isn't a big deal, but it can be annoying in situations where you're forced to hold the device for a while, as when you're watching a video on a plane.
The iPhone 4S has many advantages over competing smart phones, but its Retina display is a mixed bag. It offers a high resolution and a high pixel density, resulting in razor-sharp graphics and text. But the iPhone 4S display, at just 3.5 inches diagonally, is a bit too small compared to other modern handsets.