running it under mac OS X Leopard (latest update).
the only annoying thing is that it keeps popping up a dialog box warning it could not store "undo" date in the hard drive (the undo works just fine). i haven't tried to turn off the "warnings" in the program yet, pretty sure that might work.
another bad thing: it doesn't recognize my tablet, so the pressure-sensitive features are like disabled... it draws like using a mouse, but still works...