TidBITS: iMac G5: Up In Smoke
Apple has two problems with their iMacs - unreliable power supplies and capacitor failures. Both are described in detail by an experienced Mac journalist who had to replace his iMac. The worse news is that the iMac's automated diagnostics came up with the wrong answer -- and Apple's customer services is now pretty darned awful.
I suspect the new iMacs will be safe to buy, but I wouldn't touch any of the existing models. Shame on Apple for following the stonewall strategy; shame that it usually works.