One of the best games I have EVER played.
Assassin's Creed as an action game is great; there are complaints that it becomes repetitive and quickly devolves into buttonmashing, but I never had this problem. The combat is simple but fun. It's based on timing your button presses with the attacks of your enemies. The kill and counterkill animations are top notch too. If you experiment with different weapons you'll begin to find new and interesting ways to kill off your attackers.
Assassin's Creed as a stealth game is interesting. It's different from previous stealth games like Metal Gear and Splinter Cell because it emphasizes hiding right out in the open. Altair tries to blend in with the rest of the townspeople by walking amongst monks or sitting down on benches like he didn't just kill someone.
Assassin's Creed also has some of the best in-game graphics I've ever seen. Probably the best thing about the look of Assassin's creed is the artistic value. The work that went into detailing the buildings, the people the landscapes, had to have been painfull. Not only that, but sometimes during a kill sequence the camera comes to a low angle or maybe it swings around and the actual killing blow is obstructed by a flower or a pillar of a nearby building, and you can't help but wonder if that same sequence could be in a real movie.
I've also heard a lot of complaints about the ending, but remember that Assassin's Creed is meant to be a trilogy and the ending seems very fitting and satisfying. Like I said, Assassin's Creed is the best when it comes to storytelling and the mark of a good storyteller is that he leaves his audience wanting more. If you do a little research on the bits of info that you're given in the ending (or take the lazy way out, let someone else do the research and then read about it on Wikipedia or some other gaming site: http://www.computerandvideogames.com/article.php?id=175552) the ending becomes a whole new branch of the story. The team at Ubisoft Montreal were geniuses in the way that they chose to work in artifacts whose origins and purposes are questionable. This leaves room for the team to work in their own mythologies so that it suits their story.
The only problems with Assassin's Creed are minor skips in the framerate and sound when there are lots of enemies around. Also, I understand that not too many people want to be perplexed and made to think when they play a video game, they just want to pick up the game and play.