I ditto this. I still have a 400 mhz G4 PM that I use regularly. It's seven years old and still going strong (even running Tiger quite well!). It stays till it dies.
I keep them for 3 years or so. I finally switched back to mac after having a Gateway and Dell. Dell just died, the hard drive started clicking.
I really don't know why I moved away from mac. That was all I used in college. I can't remember the model but between the screen and tower weighed like 50 lbs.
Never again will I move away from mac...never again.