You should definitely buy this. I don't think the game got enough recognition. It was my favorite game last year. It's definitely more fun playing co-op, though. I played through the whole game with my wife.
I picked it up and played a couple hours this morning. Looks to be a short game, but so far every single minute has been riveting. Well worth the $15--and this is coming from someone who hasn't bought a day-one release on XBLive arcade since Shadow Complex last summer.