I just had some quality time with the new iMacs at the Apple Store and I have to say that it leaves a good first impression.
I'm sure that some PowerUsers(tm) will be disappointed (and they should stick to their dual PM G5), but in general use it felt very snappy. This impression is drawn from 10-15 mins use on a 1.8GHz 20" iMac G5 with 512MB memory, playing with Safari, Preview, Finder, iTunes, Garageband.... as much as is possible in a store. Compared with a dual 2.0 PowerMac G5, it's not there, but the overall feeling is that it's a step up from the Powerbook G4 models.
The screen is beautiful, and those concerned about the "chin" are obsessing too much - when standing in front of it, you're eyes are filled by the screen. The speakers are adequate, not going to bring the house down, but JBL/Altec Lansing can help you out there. The all-in-one has to be seen to be believed.
Overall, I was pleasantly surprised!
Just my 2c....