You can see my iMac's stats above. And I have something to report back with.
1. Single player - graphics set to LOW are definitely LOW. BUT BUT BUT, the gameplay is smooth and the sound is clear. Absolutely no complaints. Definitely worth it. No regrets.
2. Multiplayer - since this isn't so story heavy the graphics don't really standout so much, but again VERY smooth gameplay and that's all that matters.
Even though all the video settings are LOW, definitely not a bad gameplay experience.
-rudy
PS: the inverse of my play time with SC2 would be any Steam Source game! Those games run like crap. I have my iMac bootcamped and those Steam games run fine! But now the Mac with newly-released Steam? Poop! (I've tried TF2 and DOD:S). Torchlight plays well, no complaints there.