I am sceptical about it, because it might be just something like an add-on, and you would pay the price for a full game.
Just because it isn't
Assassin's Creed 3 doesn't mean it's a mere add on. I've been paying attention to news and updates about ACB and I am confident in saying that it will be a full game.
1. The length of single player campaign should be just about as long as the story in AC2. It may be a little bit longer, or it may be a little bit shorter. No matter how you look at it, it is still a full length game compared to both the first and second game.
2. There are many improvements when it comes to game play. There will be a few more new weapons, a polished fighting system that is promising to be more fast paced and action oriented than in the previous games, and you will have new ways to assassinate people-such as stealing horses from guards and assassinating enemies from a parachute designed by Leonardo.
3. There is a completely new element being added into ACB. You will be able to recruit new assassins who will be able to provide extra support as you play the game.
I just finished AC2 about ten minutes ago and I can't wait for Brotherhood; unfortunatly I also I want to purchase Black Ops and even more unfortunate I can only afford one or the other. My decision will depend on how good ACB's multiplayer is and how long it can keep me playing.
I have not played the beta, so this is merely my prediction. I believe the multiplayer will be either a hit or miss to players. You will find people who get bored of the online play very quickly, or you will find people who absolutely love the online play.
I used to play CoD4 all the time. I couldn't get into the online play for MW2 though. I have Black Ops, and the online play is much more balanced out. Quick scoping was nerfed and made nearly impossible to rely on, so snipers have to snipe again. The guns are more balanced. And most importantly, kill streaks are much more balanced out. Your killstreaks do not stack! If you kill 5 people, then you kill 2 with an attack helicoptor, your killstreak still remains 5.