Well, my main system is a PowerBook G3 (Wallstreet) which was released in September 1998. I originally bought it in 2001 to run Rhapsody on and to replace my IBM ThinkPad (also running Rhapsody) as my main mobile system. In September of 2002 I put 10.2 on it and it became my main system (and my ThinkPad was pushed back into service as my main mobile system). In early 2003 I installed a G4/500 upgrade and I haven't been able to justify getting a new system ever since.
This system works as good today as when I put the G4/500 in it, so I'm just as productive. Until I need something that only runs on a newer system (in other words, I need to replace software that is already working perfectly for me) I have no compelling reason to replace this system.
I don't like monkeying with a system that is running perfectly, so I haven't felt any need to go chasing after the latest and greatest of anything. I see, and troubleshoot, that type of stuff everyday. When I get home and need to do work on my system, I expect it to be absolutely trouble-free... and it is.
The most processor intensive tasks I do are in Photoshop and I almost never find myself waiting on Photoshop to finish anything, so, again, I have no compelling reason to replace this system.
Of course if someone else was paying for it, I'd get a new system right now.

But it isn't worth my time and money as things currently stand.