I have been using Leopard on my 12'' pb with maxed out 1+256GB ram. Did not go with Tiger due to the lack of time machine support. Everything is responsive. I am missing a good browser tough. I tried all G4-optimized firefox versions, but none of them can beat camino and Opera. Overall it makes a great travel laptop.
I also tried yellowdog and ubuntu, but they both lacked HW support. I would love to have linux on this machine, but it will never happen, I am afraid.