I suppose I might as well chime in as well, I love my iMac G5. I can't speak to relative performance except to say that it easily keeps pace with the dual G4 powermacs at work, though I can't recall what their processor speed is, but I know my iMac has more RAM. This is the first Mac I've ever owned, and I'm never going back, that's for sure. My old XP box was very well maintained (no virus, spam, or malware problems), but it would still go down every now and then for unexplained reasons. Needless to say, I've had no instability issues with OS X, my 20" screen has only one dead pixel (off near the bottom left where I never see it except when the screen is totally black) and is utterly gorgeous, and the OS and other software can't be beaten. Hell, even MS Office works better on the Mac. Even the fan whine that everybody keeps mentioning doesn't bother me, since my XP box's fan was unbearably deafening, whereas the iMac's blower noise can be drowned out by iTunes or game music at even low volume. The optical drive does get loud, but I expected that just based on the layout of the computer. My only "real" complaint is that since my desk faces the wall, I never get to see the beautiful back of the iMac!
I guess that was a really long way to say that I'm more than happy, I'm absolutely thrilled with my iMac G5.