Well I just got an iPhone 4 yesterday and last night and most of today so far using it.
The iPhone is so great I even find it faster than my old 3GS and it is also faster at taking pictures and pretty much everything else. This phone even has a slightly longer battery life as well which is great. The new Retina display is amazing makes everything so much more crisper and vivid. The build quality of the device is also really great and it feels very nice too. My biggest improvement to me though is the speaker phone for voice and music is great and much better then the 3GS.