Don't forget they also last longer, hold their value, and you can walk into an Apple store with a G5 iMac, that has never had the OS wiped and reinstalled, and get it tuned up and double the memory installed FOR FREE... (They did this for my mother about three months ago). Try getting tech support on a NEW HP or DELL much less a 5 1/2 to 6 year old model.
Oh, and let's not forget, Maximum PC did some testing awhile back. Best computer for running Windows? Yep, a Mac.
http://www.youtube.com/watch?v=A4WytTusemM