I love the Assassin's Creed series. They're a lot of fun and each game has built on the last, adding new weapons, new moves and a new dynamic. The settings are (to my mind at least) historically accurate enough that if you ever get to visit Italy and wonder round Venice or Rome, you'll get a strong sense of deja vu.
The problem with Assassin's Creed Brotherhood is that it doesn't build on AC2 in the way AC2 built on the first game. It's very much par for the course. Yes Rome is big and there's lots to do, yes having well trained Assassins at your beck and call is an interesting mechanic that doesn't grow old and often helps you in a tight spot, but it doesn't deviate wildly from the groundwork AC2 laid down.
Play it, it's a lot of fun and adds to the story being told over the series, but do yourself a favour and either buy it brand new from a reseller or buy it used and sell it on afterwards.