I've been snatching up deals on Steam this week too, but I think I'll pass on Mass Effect 2 for now, I'm sure that by the time I roll through some of the other games I bought, it'll be winter, and ME2 will be discounted even more for their holiday sales.
I'm eventually going to have to come around and rally some mates from here to run through L4D and L4D2 with me. I've played a bit solo, but imagine the real fun is with multiplayer.