While nice to look at, and fun for a short bit, the repetition this game boasts will not hold water with players.
The story isn't bad, but it's likely not enough incentive to keep one playing through the monotonous informative quests, which I'll get too. Most of the story centers on playing through memories taking place in a futuristic machine for purposes not quite revealed until later. The premise is nice, but the silly ending might throw some people off, and the entire prospect of this game becoming a franchise or series is kind of disconcerting, unless they change a great many things about it...
The biggest flaws this game has are: the combat, and, well, the gameplay--in a sense--what you are actually doing in the game. Repetition.
The combat is difficult to get when you first pick up the game, as it is overly simplistic and not exactly visceral in any way...later is becomes so easy it's not even fun; so, in this way, the combat--to me at any rate--was a device that you will use too often and utilize in a very similar matter throughout the game. It is nothing special, and helps to add to the monotonous feeling the game has on an overall level.
But the biggest flaw is, without a doubt, the actual doings of the game. You'll spend hours--if you put up with it--running around similar (though pretty) cities, saving citizens for no gain, interrogating silly idiots, fighting guards, walking into flag "collectibles", finding viewpoints, jumping into hay-barrels, and that's really about it. And you'll do it for the entire 15-25 hour duration of the game. Every once and a while, you'll do an assassination mission--which involves you going into a building, killing a few people, then chasing someone around a city, getting close to him, and tapping X to assassinate him. Fun, fun, fun.
So, overall, Assassin's Creed is a big disappointment in my book. The cities are nice, visually, and the climbing/acrobatic animations/mechanic works very well, and the game is quite amusing--for a short time. Then everything bad about it reveals itself. There is little to no development, and the story yields no rewards, vying instead for the "set-up-for-a-sequel" route. The repetitive gameplay and the overlong story and the completely unbalanced combat all come together to give this game a really "fuzzy", half-finished feel.
Mediocre. Perhaps worth a rent, but I shouldn't have bought it, as I'll likely never play it again....