Actually, it looks like I'll be upgrading my Mac Mini very soon to Leopard. One of the kids on my dorm floor bought a family pack of Leopard for $200 (since there's about a dozen or so Macs on our floor of 60 people). So far, a bunch of of the people who've upgraded to Leopard on my floor say that their computers seem to run
faster with Leopard than they did with Tiger.
My G4 Mac Mini is barely above the minimum specs, but someone on the floor above mine upgraded their iBook with lower specs (867Mhz G4, 512Mb RAM) than my Mac Mini, and after seeing it in person, it runs practically as well as Tiger did. (They report about 15 seconds longer to turn on, and nothing much else.) Now I have no reservations about upgrading my Mini.
That's another reason why I like Macs; the useful lifespan of their hardware is about double that of the average PC.