My 1.5GHZ G4 Powerbook had it's 3rd birthday last week.
Since new, it's been on and net searching, photo editing, document creating, email sending, cd burning, web designing, message board dwelling for no less than 6 hours a day, 6 days a week. Twice a week also, it's used to project a PPT presentation and play DVDs on a screen.
It's never been back for care and has never had a glaring issue.
Lately though, it's slowing down. Installing 2GB RAM didn't help at all since it was simultaneous with a Leopard install which it seems ill-equipped to handle.
When I get a new 17" MBP, I'll wipe this one and re-install Tiger and pass it on to my assistant. We'll see how long it can go!