This game runs beautifully on my Penryn MacBook Pro (2.6 ghz). The graphics are just stunnng beautiful, detailed backgrounds and 3 or 4-level parallax scrolling. If you like old-style Mario/Sonic-type side-scrolling platform games, ad if your Mac's specs are up to it (seems this game really demands an Intel processor) then you'll really like Supercow.
The difficulty level is "easy" for an experienced gamer in this genre, but the levels are deep, large, and very pretty. A fun game... both I and my 7-year-old love playing it together.