I'm running a beta of 10.6 on my first generation CD 2.0 GHz MacBook. No real improvements have come up for me, but I don't know how old the beta is. Under system profiler it lists the system version as Mac OS X 10.6 (10A96). I did notice a smaller footprint for the OS, but that's about it. Most third party programs crash frequently, and some don't even install. I'm going to chalk that up to an old beta on the oldest MacBook.
I'm going to backup and do a clean install of Leopard to get some of my old programs back, and put the 10.6 upgrade money into a fund to buy a new aluminum MacBook when they do the next round of processor upgrades, or a MacBook Pro depending on how much I get saved.