Personally Brotherhood was my least favorite out of the three "good" AC games (if we're not counting AC1)... the story spins its tires, aesthetically I didn't like the setting or the music as much as I did in AC2, and even though it offers some cool new weapons and the whole assassin training thing I kinda feel like they made you TOO powerful this time around. I also have qualms with multiplayer, it's a great idea but (at least in Brotherhood) there didn't seem to be enough incentive to play stealthily so you basically had a train of people running full speed at one another.
It's certainly not a bad game, but with Revelations out it's entierly skippable in my opinion. Here's the only plot point I think you need to take away from Brotherhood:
Kristen Bell's character gets killed