I got my iMac today, so here is a short review:
In brief, I can tell you that it is great! You can hear it in a silent room, but it is very quiet. My PowerBook wasn't perfectly silent, and i wouldn't say this is louder. I haven't done anything that caused the fans to ramp up yet, even though I have done things that would have gotten my PowerBook's fans going (probably at full speed, too). I think Apple's claims are well justified in the "softer than a whisper" department. You have to remember that you can here a whisper in a silent room, and likewise, you can hear the iMac too.
The speakers as expected don't have much bass, but I think they are fine for being built-in. The way my desk is, the whole bouncing sound off the desk thing doesn't really work as Apple would have intended, so I think the speakers could sound better if I rearranged things. Personally, I don't care enough to bother. If I need better sound, I'll connect my external speakers.
I think the screen is very bright, but the room I am in doesn't have very bright lights. The room is by no means dark, but it isn't an office with large fluorescent lights or anything. In order to not hurt my eyes, I had to set the brighness a bit below half.
I went with the 20" model, and the screen is gigantic. Coming from my 15.2" PowerBook, I'm definitely going through an adjustment period. It looks like my sceen has no dead/stuck pixels, but I haven't run a pixel check program or anything. However, I think if there were any, I would have noticed them by now. Viewing angles are excellent as expected. Adjusting the screen is very easy, and can be done with one finger. As i had suspected, the 20" model does look much more proportional than the 17" does, thanks to the white space under the screen.
So far I've been very impressed with performance. I have 512MB of RAM right now, and haven't bothered swithcing the Processor Performance setting from Automatic. I haven't done anything yet that would really push the iMac, so I can't comment on performace too in depth. So far though, I haven't found myself waiting for much of anything, and applications open much faster than on my PowerBook.
That's all I can think of at the moment, but feel free to ask questions.
I guess that wasn't that short of a review after all.