Nice, does it develop (in a useful way) the storyline with Desmond and all that. Or is it just sidetracking and useless stuff waiting for AC3? In other words, if one were to skip the game and only play AC3 as the next game, would they be like "omg why is this character gone?" or similar questions.
For the most part, it's not a HUGE advancement of the storyline... but it ends with a cliffhanger moment that will (presumably) affect the course of events in the present-day situation with Desmond & Co. The thing is, there's some ambivalence to that moment, so you're kind of left wondering "did that really just happen??"
I'm assuming they'll recap that moment at the start of AC3, given that it's a pretty huge development, so you could probably skip over AC:B without being completely lost, but it's hard to say. However, it's not a thing you the player have any choice about or any involvement in through the game - it's just part of the final cinematic. You could just watch that cinematic on YouTube and you'd be up to speed. (But if you're thinking of playing the game, don't spoil that moment - it's about the only thing that makes the ending at all climactic.)