Dang, I'm actually considering getting AC2, but I've already beaten it, but just have like 4 achievements left (feathers and some other stuff). Hmm, maybe I'll just rent it or something, don't know if I can justify 10 dollars on a game I might not play again (but I sure as heck thing it's an amazing game). I might get the rest of the DLC for all the other AC games though plus some more arcade games.
With that said, anyone on the fence about AC that hasn't played it, buy them! AC1 is awesome IMO, but if you play it after any of its sequels, you might want to gouge your eyes out, it will be very boring, but I think it might be worth going through it and experiencing the game yourself.