My first impressions:
When I first held the iPhone 4 it was hard for me to believe that it was the follow-up to the 3G/3GS. The physical design is so much better than on the 3G/3GS; the simplicity of the glass and stainless steel is head and shoulders above the plastic/chrome bezel of the older iPhones. Simply put, it feels like one of the top smart phones on the market, not like a toy. On that note, the switches and buttons feel more solid and purpose built (I doubt anyone will be breaking off the volume switch this time). It also feels a little smaller, probably because there is less bezel; likewise, the square shape of the device makes it seem more elegant.
Then you turn on the device, and you see the screen, and all you can say is that it is truly fantastic. I listed to Jobs reiterate how good the Retina Display was, how sharp the resolution was, the pixel density, and so on, and truth be told I figured it was all marketing hype. But I can assure you it isn't. I've never seen any device with a screen this sharp. It's truly magnificent, and combined with the extremely sharp color spectrum, it's a pleasure to look at, and to use. In fact, everyone I know who owns a 3GS has trouble going back to their device; when they use the 4, they can only describe it as akin to lifting a fog from the screen.
The speed of the device is great (coming from a 3G, I notice quite a difference). It's clear that the new processor is an excellent fit, especially when playing taxing games like Grand Theft Auto (I had to download the demo and give it a go). That being said, all of that performance can be tough on the battery. At first I was getting pretty terrible battery life (granted, I was using the phone quite a bit), but after a few charge cycles, the battery seems to be calibrated nicely, and I'm getting battery life comparable to my 3G.
I've also noticed that the speaker is much improved over the 3G. The sound is so much more crisp, and the volume seems to be louder as well. The camera is absolutely stunning, and the flash is a very welcomed addition. Face time is pretty keen, although I wish that it were available for 3G calls. I'm also happy to report that I get better signal strength with the iPhone 4 (especially in my basement which is notorious for dead zones). And, unlike so many claims, I cannot reproduce the supposed reception/antenna problem.
In all, I would say that the iPhone 4 is head and shoulders above its predecessors. Not only does it look stunning, but it performs equally well. I really couldn't be happier; it was worth the wait!