Ideally coming into this you will have completed Assassin's Creed 2 Assassin's Creed 2 : Complete Edition (Xbox 360)
. However it's not the end of the world if not. The game has a neat way of recapping the story from the 1st and 2nd game right at the beginning of this.
Those that enjoyed the 2nd game will enjoy this. It's very similar however I'd say it justifies being a whole new game (there's a lot of content) rather than being called an add-on.
Multiplayer is great fun and refreshingly different in comparison to 1st person shooters. The main "Wanted" mode is such a great idea. Haven't got round to doing the team modes yet.
Just a quick recap. You'll love this game if:
You enjoy sneaking around all stealthily
Played Assassin's Creed 2 and enjoyed it
Love a story line full of mystery and conspiracy
or as an excellent pre-cursor to visiting Rome (it's very faithfully recreated in the game)