It didn't take long to outgrow my first-generation G4 mini. The internal hard drive is a 4200 RPM 2.5" model that's nowhere near fast enough, so I switched everything over to a proper 7200 RPM 3.5" drive in an external Firewire enclosure. It solves the speed problem, but I can't put the Mac into sleep mode without leaving the enclosure on, thus defeating much of the purpose. I also got one of the models with the wonky DVI-D output, so it never quite worked right with my monitor. About once a day, it would go blank for 2 seconds. (Good thing I didn't use the mini for games.) I could have gone on living with such irritations, but this was the last straw: I've outgrown iPhoto, and the mini won't run Aperture.
Yep, I've already hit the point where the apps I want to run are only available for the new hotness. Under normal circumstances, a Mac's useful lifespan would be longer. But I got caught on the wrong side of a Major Architectural Change, and the G4 mini was already one step behind the G5 desktops of the day.
Thus, after only 1 1/2 years or so with the mini, I'm moving on to the hard stuff. The 24" model, with 2.16 GHz of Core 2 Duo goodness, 2 GB RAM, and the graphics upgrade. I may even pick up Parallels, although I don't need another XP machine.
Although, now that I think of it, I found my OS/2 Warp 4 CDs as I unpacked the office…