I just finished installing the latest version of Mac OS X (10.4, Tiger) on my PowerBook G4. I backed up all of my data to an external FireWire drive and did a fresh install. It certainly feels faster, especially Safari.
Right now I only have one serious problem with Tiger, Desktop Manager is broken. There is a development version that at least runs, but some of the features don’t work and it causes some other strange things to happen (like the dock disappears some times). It also crashes from time to time. I’ve generally got nine or ten virtual desktops going, I REALLY want to see DM working again.