I recently upgraded my 2008 MacBook White from 10.5.8 to 10.6.6 and my reaction after using the upgraded OS for five minutes was "boy, it sure is snappier; I guess I should have done this a long time ago."
I hadn't been expecting the upgrade to improve performance (the only reason I did it was to get the new Mac App Store), so I did some research on Wikipedia and it said that Snow Leopard had large portions of code re-written to improve performance, something I hadn't really been aware of.