Just beat the main storyline yesterday.. If you like AC2 you'll love this one..
It's nothing astounding, but it is still AC and it is still awesome. Without giving anything away.. the story is pretty uninteresting until the end and it leaves you with a massive cliff hanger and I literally just said wtf.. why are the credits playing already?
The multiplayer is lots of fun too.. really intense and gets you to plan your kills instead of running around like a chicken with its head cut off like you do in CoD. Highest score off one kill I've done so far has been 600.. 750 is the max and i still cant get to that one.. if you just run up and kill the person its 100.. so it forces you to do something fancy to get lots of points.
If you aren't sure you'll like it.. after playing it gamespot's video review does it justice. That should help you decide