My 20" iMac G5 is loud to me. Its a humm when it starts up, but after that its fairly loud. My CPU temps are always around 140-150 with the settings on Automatic, and 150-160+ when the settings are set to Highest. My fans never seem to go back to normal after cooling down. My iMac was extremely quiet when I first got it about 2 months ago, but now its too loud for me. I'm strongly considering selling it for a 15" or 17" PowerBook.
What seems to get really hot is the powersupply in mine. I've seen many reports of pushing, or even spewing capacitors on iMac G5's. I hope mine doesn't start doing that....
I did reset the nvram and it was back to the way its supposed to be, but after it got hot again all the noise came back. I shouldn't have to reset the nvram and then press the lower button on the logicboard everytime it gets hot.
Other than that, its an awsome computer. It has a beautiful screen, more than enough power for me, and its a real space saver. It also looks great, especially sitting on my glass corner desk. I must also say that Tiger absolutely flies on this. Its twice as fast as Panther was.