As I mentioned already I picked up an iPhone 4 and I have to say I'm quite impressed. OK so the OS is identical to the one on my 3Gs but the redesigned case is excellent. Not that I didn't like the old 3Gs case but it was plastic and that did tell after a while. The new glass and metal of the iPhone 4 is nice and sturdy. The few buttons on the device have a more solid click to them and remember my 3GS was only a month old so it's not like its buttons had worn. The only structural problem I had with the original case design was a crack on my older 3G where the USB connector plugs in. The plastic developed a hairline crack about a centimeter long, it never grew or got worse but it was there and it did make the case feel cheaper. The new connector is surrounded by metal so hopefully that wont happen again. The home button on the 3G had started to go as well, it didn't always click properly. I suspect some dirt or fluff was wedged under there but I couldnt get it out. I'll have to wait and see if the 4 develops a similar problem in a year or so but with the home button being so important for multi-tasking I hope Apple took special care with the design this time.
The retina display is, well, it's an iPhone display. Sure, it's nicer and smoother, less pixelated when I hold it up to my eye but it is solving a problem I didn't know I had with a solution I cant really notice. It's nice to see Apple moving the technology along but I wouldn't have bought a new phone just for that.
The camera is a big improvement, especially for video. Pictures and video are cleaner and more usable. It might be the best camera phone around but I wont be taking my SLR out of it's bag, or even my point and click. The point and click might get used less but I still like having the extra controls and shutter button. The iPhone camera flash might make a useful torch when I drop my keys but I have yet to use it with a photo so I don't know how well it performs as a camera flash.
All in all I'm happy but my fiancée is happier because she got the 1 month old 3GS to replace her first gen iPhone.