Yes I used Leopard for two years, and for me, it was absolutely stable and perfectly fantastic. Snow Leopard, on the other hand, introduced a few minor bugs, and slowed down a few things. I really preferred the way Exposé and Spaces worked in Leopard instead of Snow Leopard.
I guess it's down to personal experience, but I always felt that Leopard was polished to perfection while every subsequent OS X was a bit rushed and the buggy new features always outnumbered the fixed old features.
For example, in Leopard, there was a major emphasis on gestures, and they worked very well. In Lion, these gestures were changed, but in a way that broke certain gestures (like 3 finger swipe) and introduced inconsistencies (the swiping direction was "in the direction of content movement" for most gestures, but remained the opposite for others, like 3 finger swipe).
Leopard also introduced Spaces, and it was absolutely perfect (for me) as it was then. Snow Leopard broke it somewhat, by introducing jerky animations and some minor glitches. Also, Exposé was changed to no longer preserve relative window positions and sizes, something we had to wait for until Mountain Lion to get fixed.