Finished Mafia 2 last week, thinkning hard about picking up Starcraft 2, but I suck ass at multiplay RTS games. Heard singleplay is good though.
I would highly recommend Starcraft 2 single player campaign. It seems kind of pointless at first, but the missions come together at the end and make an excellent story, which of course sets up the next installment.
I havent tried online either. I have played a lot of RTS games, but still manage to get my butt kicked online, and I think SC2 would be even worse than most. Actually, even the skirmish AI is brutal, but again, I really liked the single player campaign.