My systems are:
G3 tower: Probably about 7 years old at this point. Still works fine; we use it for studio recording and running it on OS9. We just dropped in a OWT G4 processor @ 500mHz (a bump of 150mHz and altivec from the previous system) and the little baby still cooks! For our current uses, I expect to get another five year sminimum out of it, if it ever needs replacing at all (depends on what the new technologies in retail music become, like DVD audio or what have you).
G3 ibook: 3 years old at this point. Still runs smoothly. One of our interns uses it regularly now.
G4 tower: Runs the whole company. An original sawtooth @ 400mHz, bought it in 2001, so about 4.5 years old. I just bought a processor upgrade from Sonnet to bring it up to a gig, but really it barely needs it (it started running slower with more than 4 newer apps running simultaneously). With the upgrade I expect it to easily carry us through the G5 era and into the intel chips, as we plan on skipping this gen...
G4 ibook: Bought it refurbished a year ago. 1gHz ibook. Flawless, will probably be my personal computer for the next 3-4 years minimum, then an intern machine after that.
