Day 2 with my iphone 4; still no issues.
Battery improvement is very significant, makes a huge difference to my usage. I'm no longer thinking, will watching half an hour of video kill my battery.
The wifi reception seems dramatically improved. Compared to my iphone 3g, and my panasonic toughbook, i'm seeing an additional 2 networks i never knew existed.
The screen is as big a step up as they quote, unfortunately, most apps haven't been updated to make use of it. The most obvious place is looking at app logos, the icons that have been updated are a huge step up from the ones that haven't.
The camera is pretty good, although i would prefare if they would bring out some form of image stabilisation, it needs it.
Build quality: Perfect so far. Really does feel a lot more classy and well built than any other phone.