A little bird *very much in the know* once told me about a number of skeletons in the G5's closet, particularly the first generation (1.6-2.0Ghz) models, but that's about all I can say. It's probably just enough to warn anyone considering them that they're essentially exotic UNIX-workstation technology done on a shoestring budget. (At two grand-plus they may of seemed expensive, but a SUN Sparc machine in the same speed class would of been in the five figures.) And like anything done on a shoestring budget, there are... compromises.
Basically, they're the poster children for why Apple had to switch to Intel. Take that for what you will.