ACB is just as good as AC2, because it's essentially the same game, but it's certainly worth playing
ACB has improved on AC2 in many ways. There are no more a few cities to travel, instead there is only one very big one with different environments such as outskirts. Some missions will feature other places and towns that you will travel to, but those are restricted only for those occasions and are not part of the sandbox.
The city is plagued with events and missions to undertake, perhaps even a bit to much, so that you will never run out of things to do. The newest feature is Assassin recruitment that you will unlock later on in the story. You can recruit up to 10 team mates that will assassinate and fight your targets. They will aid you mostly anywhere with just one press of a button.
Your recruits can be dispatched to undertake certain missions across Europe, which will earn them experience and eventually new abilities. Assassin's Creed never was a particularly challenging game, thanks to the easy counterattacking, and this feature makes things even easier.
All of the missions now feature optional secondary objectives, which can be quite fun to pull off because they will be the only challenging aspect of ACB. But since you don't get any worthy rewards for completing them, it might seem as pointless frustration and waste of time.
ACB is the first AC title to include online multiplayer. The premise is simple, you are given a target to assassinate, but someone else has also gotten you as their assigned target, so you have to watch your back at all times and focus on your target at the same time. The city is filled with lookalikes to deceive you, but a handy compass will help you locate your target.
There are a number off ways to pull off assassinations, and the stealthier you perform the more points you earn, so rushing in to stab your target isn't a good option. You cannot kill your pursuers, but if you notice them you can stun them, making them lose their contract and move on to someone else.
Sadly there are only about 10 maps to play on, and many of them are the same map with a different time of day, that means even less map variety.
As you play you unlock additional abilities and perks, such as sprint boost, smoke bomb, disguise, poison etc. Online multiplayer is a lot of fun if you're patient, because once you land a sweet stealthy combo assassination you will be well rewarded. It's probably not for everyone, but it's great change from all other army shooters we keep getting these days.