I'm delighted to confirm that, on the iPhone 4 anyway, iOS 5 is as smooth as can be.
There were a lot of niggles with the UI kit on 4.2 onwards, but the whole UI kit is butter smooth in 5.0, it's superb. The main niggles, as those familiar with my posts in the 4.3.3 animations thread will know, were that when you opened third party apps for the first time (after install or a reset), the launch animation would not run fully or at all, and look very untidy. When you returned to the home screen and relaunched the app, the animation ran in it's entirety. This has been rectified in iOS 5.
Also, if you had a full page of apps (all 16 spaces used), and you opened the multi-tasking tray, it would stutter a little. Now, on iOS 5, it's smooth as can be ... iOS 5 is a joy to use, it returns the iPhone 4 to the way it was on 4.0, if not better. I would imagine that these optimisations will come to the 3GS also.
There were a lot of niggles with the UI kit on 4.2 onwards, but the whole UI kit is butter smooth in 5.0, it's superb. The main niggles, as those familiar with my posts in the 4.3.3 animations thread will know, were that when you opened third party apps for the first time (after install or a reset), the launch animation would not run fully or at all, and look very untidy. When you returned to the home screen and relaunched the app, the animation ran in it's entirety. This has been rectified in iOS 5.
Also, if you had a full page of apps (all 16 spaces used), and you opened the multi-tasking tray, it would stutter a little. Now, on iOS 5, it's smooth as can be ... iOS 5 is a joy to use, it returns the iPhone 4 to the way it was on 4.0, if not better. I would imagine that these optimisations will come to the 3GS also.