4.0 was really fast on iPhone 4. It was painfully slow on iPhone 3G. The difference was so noticeable, I even started to wonder if Apple was purposely driving upgrades. Now that 4.1 is out for developers, I can put my paranoid suspicions to rest- 4.1 restores the former glory of iOS 3 to older 3G handsets and then some. The UI is snappier than ever, gone are the random graphical glitches, and apps load quite a bit faster. As for iPhone 4, iOS is smooth, responsive and stable. Performance has improved ever-so slightly, but any enhancement in that area is always welcome.
This one sort of came out of left field but is an excellent addition to iPhone 4's already terrific camera. In some quick home tests, I've already seen that HDR photos generally look better than non-HDR ones. Obviously the results really shine when there is a lot of light and a lot of shadow in the frame, but I'd say roughly 7 times out of 10, turning HDR on improves picture quality. The only downside is it takes a couple extra seconds to take the photo (as it's actually taking three photos in rapid succession), but this is a very minor gripe.
Game Centre has some serious potential. Right now, there aren't any games to try it with, but I really can't wait. It brings to mobile gaming what XBOX Live has to console gaming in a lot of ways (and doesn't cost an arm and a leg). Apple is really pushing iOS as a legitimate gaming platform, and Game Centre is the next step. Though I've never used it, from most accounts OpenFeint is awesome and did it first. But with Game Centre being so integrated into iOS (APIs included in XCode, Game Centre on the home screen, being able to use your existing iTunes account), I imagine a lot more people will adopt Game Centre.
Now that I've been using the GM release for the past 24 hours, I can safely say - iOS 4.1 on iPhone (including the 3G) is the most solid, responsive iOS release to date. With some cool new features to boot.