Leopard is running much better for me than I thought it would on my 867 MHz 12" PB with 640 MB RAM and 100 GB HDD. Spotlight takes a couple seconds to load my search and coverflow takes a couple seconds to load previews, but otherwise only small headaches (e.g. locked up during install while installing some printer drivers, epson I think). I haven't tried Time Machine yet, and iTunes, as well as Finder have quit on me a couple times, but no total system crashes. I did do a clean install, btw.
edit: Actually another headache was that Panther synced and connected to the internet just fine with my Motorola V262, while Leopard didn't recognize it when I plugged it in. I played hell figuring out how to get it to work, but now that it does it works well.