I installed it last week on my PB G4. It was really slow at first, but after Spotlight finsihed indexing my drives, then it was fine. Only two issues right now: Dashboard has to wait a second or two to re-populate widget data and Spotlight indexes EVERY drive so if you connect a new external drive or iPod, it begins indexing and that slows the system WAY down.
Overall, it frickin' rocks. Spotlight is the best thing since the mouse in my opinion. You don't have to organize any more because it indexes on the fly and it gets smarter over time.