The quad actually has very good reliability scores amongst all G5s, even better than some of the early air-cooled models:

http://www.macintouch.com/reliability/pmg5.html

G5s are still iffy by comparison, but the quad did have most of the kinks worked out, and the dual 2.3 A/C is quite solid too. Those are the only PM G5s I bother with.