A nice surprise for me is that I've noticed that the back of my iPhone4 isn't showing fingerprint smudges like I expected: it's very minimal compared to the back of the my 3GS which always looked yucky. Does the back glass have the oil resistant coating as well as the front?
The battery life is improved over my 3GS. My usage time was 4 hours and 45 minutes when run down to 20% vs 3 hours and 30 minutes on the 3GS. I've kept the settings the same on both phones. This is a very nice improvement, but I haven't seen the amazing results that some have reported.
I love the way that IP4 feels in my hand: it's such a nice, high quality, feel.
Multitasking is great: I have to keep reminding myself that's it's available and to double click the home button.
I'm so glad to have a better camera with HD video in this phone: much better pictures.
The screen is wonderful. I think that I would have upgraded if the retina display had been the only improvement (well, maybe not, but this display is terrific). Having easily changeable backgrounds without jailbreaking and using winterboard is nice: the backgrounds dress up the display nicely.
I'm in the Dallas/Fort Worth area, so I've had no reception issues. I remember when I got my 3G in 2008, I would have 5 bars while driving down a road, pull into a parking lot and get "no service", pull back out onto the road and get 5 bars. At that time, 3G reception in the Apple Store in my area was awful. AT&T fixed things and now 3G has been good here, generally. Of course, their corporate headquarters is here, now.
I was in Durant, OK on Saturday, which has no AT&T 3G, and it was maddening. I saw a lot of 3G iPhones there, but it would drive me crazy to be on EDGE all of the time.
Anyway, I digress. I love my iPhone4!