An awesome addition to the Assassin's Creed series!
The Good:
1) The combat system is better and more efficient. I love when you kill a guard you can target another guard and perform an instant kill on him, such as a quick stab into his neck. You can do this over and over again until before you know it, guards are lying around you and you didn't even get a scratch. It is a lot easier to dispatch a large amount of guards quickly and fluently which is key in many combat situations.
2) New weapons. There is a larger variety of weapons in ACB, such as the crossbow and the bastard sword. With the option to purchase the large weapon sheath, you can equip large weapons as your primary weapon, such as an axe or large two-handed swords. The crossbow becomes useful in many situations, such as when you have to kill a distant guard silently. Poison darts are also among the many new weapons, in which you can poison your targets from a distance.
3) Improved economic system. In ACB, you have to spend money to make money. Once you destroy a Borgia tower, an area of Rome becomes free from Borgia influence. You can then renovate shops and monuments which brings in money. From different renovated shops, you can purchase weapons, paintings, medicine, etc. You can get your share of Rome's income from banks for your own use. In ACB, the economic system is more useful and efficient.
4) Assassin recruits. Now that Ezio is the leader of the Assassins, he can train assassin recruits. Te recruits are a really cool addition to AC. They assist you in any situation, whether you need support in a skirmish or you need them to eliminate a guard that is endangering you of desynchronization. They can be sent on missions across Europe, from which they gain experience. With there experience points, you can upgrade your recruits' armor and weapons. In combat, assassin recruits are very useful. They too can easily dispatch enemies and it is cool to watch your assassins in combat. However, your recruits CAN die, but they can be replaced. As long as you monitor your recruits' combat situation, (whether they have too many guards on them, if you need to call in more assassins, etc.) you shouldn't have to worry about them dying.
5) Online multiplayer. Online is yet another very cool addition to AC. There are many characters you can play as (such as the prowler, the doctor, the smuggler, etc.) and 4 game modes you can play (such as wanted). Online is centered on being stealthy. I only play wanted because I don't like the other game modes, but basically in wanted you're trying to kill a target while someone is trying to kill you. You can make your own set in which you have different perks, abilities, death streaks, etc. Online is fun, but not addictive like COD online. Still, it is a revolutionary kind if online gameplay that is a new gaming experience for everyone.
The Bad
Honestly, there isn't much bad in ACB... all of AC's past issues have pretty much been fixed. Besides the fact that story mode can be boring after you beat the story itself, there is not much to complain about.
This game is a fantastic to have in your game collection. I would strongly recommend buying it to fans and noobs of the series alike.