Leopard, to be honest is fairly buggy for me. Few months down the line I get sleep issues (every now and then everything gradually freezes after wake, first some applications then the rest of mac), time machine errors for no reason, network drives get disconnected every now and then under heavy load with most vague and pointless messages - I go to network in finder, list of computers there, but no visible shares on any of them, I go to any windows machine - of course everything works. Just little oddities all over the place.
I would even go as far as claiming I haven't had a single week with Leopard without something crashing, freezing or just ceasing to work after a while on my mini. And I've used previous OSX's on various macs for years without any issues.