I'm ashamed to admit that I bought Portal 2 (over a month ago) but still haven't actually played it yet. I hear that it's amazing though. But more importantly, I don't think you'd be asking this question if you had played the original Portal (@516). It's being offered for free until tomorrow; get it! http://store.steampowered.com/app/400/
portal 2 is awesome! if you liked portal then you should def get this. its a full game this time around and not just 2.5hrs of game play. i've finished in single player mode, but only played maybe half of the co-op mode. i def recommend it :)