I have one of the first version of the iMac G5 (see signature), and I simply can't find fault with it. Its incredibly well engineered and a real pleasure to use. My sister has also bought a similar model, and is very pleased with it.
The screen is bright, crisp, wide and easy on the eyes. The design is smart and easy to live with. The noise level is very low, even by Mac standards. Performance is great, even in the more demanding applications. The slot-loading drive is a delight to use and doesn't require the user to even look at what they're loading.
The speakers are not very loud, though they're more than enough for playing games, chatting or listening to music when you're sitting at the computer, for anything more you'll want to hook up external speakers. With an optical and analog line out, as well as Airport Express as an option, this is no real limitation.
Overall, its an incredible machine. You really get the impression that Apple have thought of everything when they were putting this one together.