Finished the story today. Doing a fair bit of side missions (60% total Sync) it took me 16 hours. I would love to go on about the game and how good it is, but I have to agree with everything Exiled_Rover just posted. The Desmond Story progressed a lot more then I thought it would in a game that wasn't supposed to be III, but I loved it and now can't wait for when III does come out.
If you played AC II thoroughly and enjoyed it, you're going to love AC:B. Everything about the game (story mode wise) is better.
Multi player: So much fun. Don't find myself getting too mad when killed, and loving the tense anticipation the action brings. Big Fan. Recommend buy for anyone who likes this type of game.