I have my iphone 4 for almost one and a half years and it was starting to get sluggish. It was running on IOS 5.0. This resulted in long load times for the camera app and even jittery transition effects when changing homescreen pages. Now I have the 16 GB model and I thought it was due to it being almost filled to the brim with apps and music. But after freeing up 2 gigs it still had the same issues. Now I just upgraded to IOS 5.1 and everything works a smooth as it was in the beginning and even my camera app launches in about 2 seconds.
Bottomline: I like the IOS 5.1 upgrade!