I want to provide a slight update on my experiences. I agree with what people are saying on the blizzard forums and here: the game starts off tolerably, and progressively becomes worse until it's literally unplayable. I'm pretty confident that the people who claim the game runs fine are not very far in the game at all. If you haven't fought the skeleton king at the very minimum, you probably don't understand what we're saying. If you've made it to act 2, things become even clearer...
That being said, although I haven't tested it personally, I've read that you can get 20-40 fps with beta drivers on the boot camp side. I'll be trying that tonight, and if it's true, boot camp looks like it's the only way to go if you have a 9400m, for the time being that is.
If you're planning on staying on the mac side, in its current state, I can only say the game is not worth your money. The mac side seems to be having universal frame rate issues anyway, regardless of gpu, if the blizzard forums are to be believed.
EDIT: The beta drivers that I tested make a world of difference. Even in Act 2, the game is playable (which is a step above tolerable for me), assuming you play the lowest settings and 800x600 resolution. More importantly, the frame rate seems stable, which has been my biggest issue so far.
EDIT 2: Alright, I beat the game on normal. Believe it or not, Acts 3 and 4 (generally) run smoother than Act 2. There are some slight pockets in Act 3 that are a problem, but Act 2 is by far the most problematic Act, and even then, it's only in certain locations (this is all under bootcamp). Generally speaking, the game is playable...on bootcamp, so if you're willing to do that, and don't mind playing at the absolute lowest settings, it's fine. I personally don't recommend buying it otherwise, because even if you can get through normal, Nightmare will just make you frustrated and mad at the game.