After you put some time into it, MAG makes other console FPS multiplayer modes look kinda... dated. I think it'll take a while to get off the ground, but it's legs will probably be abnormally long.
That said, RPGs usually sell some huge portion of their LTD during their first couple weeks, or even first week. They have a horribly long used lifespan at used game retailers -- which kills their legs in a major way. MAG is the opposite. I doubt finding used copies will be even close to as easy.
If MAG wasn't a new IP, I'd say that it'd probably come out on top, if we revisited this topic, on Jan 26th, 2011. Jan 26th, 2010, seems like no contest for Mass Effect 2, though.