I am running Tiger on my PowerBook G4 (specs in signature), but have a leopard disc when I feel the need to upgrade. I have leopard on both of my powermacs as well (specs in the signature)
Well again with my trusty g3 i can boot into OS9 to play warcraft 2 when i get nostalgic but other than that...i wouldn't say i use it so...take what you want from that.