I first tried the version 2.2 but didn't work on OSX Leopard, then I tried the latest version, 2.4.2 and it seemed to work very well.
If I couldn't control a certain game I just went to the game-properties and chose keyboard and everything worked fine.
I loved it!
Every game worked fine until I loaded Frogger, which I couldn't get to response in any way, from then on things no longer worked.
River Raid for instance: when I start a new game the plane steers right and crashes, I cannot control it. Very weird because I hadn't changed any settings.
No matter what I do, nothing works.
Pole Position, I can press left or right as much as I like, the car can no longer be controlled.

I deleted Stella's preference file, got rid of Frogger and even downloaded a new River Raid en Pole Position Rom.
I cleaned all user cashes, repaired the permissions, I even did a virus check, restarted my iMac numerous times and re-installed Stella.
Nothing!!
It won't work.
Crap!