Assassin's Creed sets the bar high for all action games with outstanding visuals and game play, and with minor setbacks.
Assassin's Creed's game play consists of you, Altair, completing investigations in three lifelike cities (Damascus, Jerusalem, and Acre) to give you clues about the Templars you are to assassinate and their whereabouts. One of the major setbacks of this game is that the entire game, or ALL the assassinations, consist of doing investigations, then assassinating. There's basically zero creativity there. However, the cities are so amazingly real and stunningly beautiful, you will get over it VERY fast. Each city consists of thousands of people walking through the streets, guards who will interact when they see you committing a dastardly deed, and much more.
Aside from the visuals and game play, the game's soundtrack consists of orchestrated songs that really fit the part of this game. I can't remember a time when I heard one of the fight songs and thought "What????" because they all flow very nicely. Songs will change depending on the mood of the game, for example, upbeat songs will play when you're fighting or being chased.
The combat in Assassin's Creed is also a pretty strong feature of the game. It's not so easy that you will never have to worry about dying, but easy enough that you can take on a couple of guards without worrying. The game gets increasingly difficult as you progress however, and eventually will become a very difficult game, especially when assassinations will begin to consist of jumping on boats loaded with mercenaries.
Overall, Assassin's Creed is a very strong achievement, and should be on every gamers shelf.