Leopard +1.
I've had nothing but stability issues with Tiger on my work G5. Finder crashing everytime I have more than one copy operation going on, print jobs processing and files being PDFed or I CTRL+click on an item in the Finder. Speed sucked, copy time sucked, issues with our server, on and on.
Leopard brought sanity. Been using it at work for a little over a year. Finder has crashed maybe twice, even with four or more copy operations and a bunch of other stuff going on. No issues with our server and best of all - it's fast.
My experience(s) anyway.