I bought iMacs for myself or others 12/04, 11/07, 7/08, 6/09, and 11/09.
The 12/04 (17" G5, when they had just come out) just had the power supply capacitors fail. They are being replaced. No other failure.
The 11/07 had its display fail in warranty. No others have required repairs, and they are all still in use by various people. AFAIK all are powered on 24/7 and sleep when not in use.