Brilliant and ambitious game design, slightly flawed execution.
Assassin's creed is set in the Middle East during the Crusades. Basically, you play as an assassin named Altair and go on missions to kill people. The way you do it is that you go to a town, and try to gather information on your target, and then plan a way to strike. However, that's pretty much all you do in the entire game. And believe me, it gets boring after a while. This is not a game like Oblivion that you can play for hours at a time and not run into repetition. You should probably take a break and do something else after each mission.
Another small gripe I have is with the cutscenes. There are long sequences of dialogue between Altair and the other characters at regular intervals, in which the player can do nothing but pace around in circles like an idiot. The dialogue itself is about as straightforward as Tolkien, so you probably won't listen to most of it the first time.
The story is actually pretty good if you learn to tolerate the long-winded cutscenes. But the voice acting is a mixed bag. I thought that it was very good, but some people might be put off by the fact that Altair speaks in an American accent. But personally, I thought it was a good decision by Ubisoft to do this, because Altair's background is mostly unknown. Some people may disagree though.
However, there are several areas in which this game exceeds. The acrobatics are very fun to perform. It is an absolute blast to watch Altair run, leap and climb across the rooftops. The combat, although limited, can be fast paced and fun if you play it right. The main assassinations are brilliantly designed for the most part, although many of them have fairly long cutscenes to go with them. But like I said, this is a small problem.
Assassin's Creed is an amazing game in my opinion, but it's not for everyone. I recommend it to anyone that likes good games and has a good amount of patience.