This game is excellent, but not quite as good as I expected. You play as Altair, a member of the Hashshashin (a group of assassins that performed politically motivated murders during the 11th to 13th century).
Although you are playing as Altair for almost all of the game, you are really playing as Desmond, who is an a machine, the animus, living out the memory of his ancestor, Altair. It might sound confusing, but after a couple of hours play you'll understand.
It's a third person stealth game sprawling cities and wide open countryside to explore. The game has got bad reviews from some sites, but there is a reason for this, which was discovered on another site. If you just play the game to beat it as fast as possible, the game will be boring, but if you play it at your own pace, how you want and do what you want you will thoroughly enjoy it.
The combat system is superb and incredibly well synchronized and the free running is outstanding, anything that looks like it could be climbed, can be climbed. Oh, and the graphics are breathtaking!
Overall this is an excellent title that really shows the graphic capabilities of the 360 and will provide many hours of enjoyment. It could do with a few more side missions and perhaps a little more variety among the said missions, if it wasn't for that, it would be 5 out of 5.