Don't misunderstand, I've seen some successful pi cabs as of late, but they run stuff like pacman, which I can run on a 5 year old cell phone. If you are into those sorts of games you'll be fine, but if you want to run something even as recent as cps2 games, you'll run into issues unless you run a really old version of mame.
I think Haze pretty much spelled it out in another thread, but then again he's understandably a bit of a purist.