After a week with my iPhone 4, I'm generally pleased. The "Retina" display is lovely to look at. I'm not so sure of Jobs' claim that 326 ppi is beyond the limit of human resolution. I still can see the pixels, and my eyes aren't even that good. But it's far and away the best display I've ever seen.
I like the "snappiness" of the device. There's no lag when flicking through apps or pressing the Home button. My old 3G had a slight lag, and I've noticed a lag with Android devices. The iPhone is very responsive.
I haven't had any issue with the proximity sensor. It seems to be working fine. The reception bars drop when I pick the phone up, but I haven't lost a call. In fact, calls generally sound far better on this phone than my old 3G.
Apps look great, but some are having update issues (crashing or having bugs introduced). I've had both Mail and Safari crash. So I think iOS 4 needs a bit more work, and that may by why the imminent update has been delayed.
The still camera isn't as good as I had expected, but the video camera is better than I had hoped. On my travels, I used to carry both a still and video camera. The quality of the iPhone 4 is decent enough for my purposes that it can be the only recording device I carry with me from now on. And that alone amazes me.
Like others, I have a small scratch on the front glass. I keep the phone in my pants pocket and never have anything else in with it. So that surprised me. I was upset that Apple swapped the speaker and mic placements. I have two Griffin AirCurves that I use a lot, and the new iPhone renders them useless. I'm also surprised at the lack of case choices currently on the market. I ended up with a Rocketfish case, which is okay except I have to remove it to use the phone with my Belkin car adapter. My 3G cases worked fine with it. Outside of those minor complaints, I'm happy with my purchase.