The game was pretty short, so I'll try and keep my review the same.
It looks very good, but some of the game mechanics are frustrating; in spite of a slew of fight moves, the counter is the only one that's useful/needed, making for some shallow combat.
There are some rather bad choices of action buttons, the details of which I'll spare you; the intention may have been greater immersion, by requiring a certain degree of attention on the part of the player, but the effect in my case was frustration leading to quite the opposite.
The missions are essentially consisting of:
(1) run around the city climbing towers or get some hint from city's assassin union rep about location to get info on assigned target;
(2) gather two or three pieces of information through one of three possible avenues: beat it out of someone, steal it out of someone's pocket, get it by killing on behalf of another assassin that will in turn tell you something;
(3) run around the city to find your mark;
(4) kill that mark (in most games this would be a boss battle, here it's easier than step (2) above) and return to your local union rep and then union leader for the pat on the back.
Then take this mission structure, and multiply it by nine, changing the location. Then add two more missions at the end that don't fit this pattern; and just as you start to think "great, finally something different" it's over.
Here's hoping the soon to be released sequel Assassin's Creed II
won't just offer more of the same.
For replay value... there are some things you can do, strictly for obsessive-compulsive gamers - like collect hundreds of flags, climb every possible tower, rescue every citizen in need, kill all 60 templars; but there is absolutely no need to do these things as they will not help in the main story in any shape or form; on the 360, the achievement points would justify completion, on the PS3, there is no trophy support (inexcusable; though trophies weren't available when released, there should've been a patch enabling them at a later date).
It is almost worth a rental, as you'll finish it in about 3 days and then never think about it again. Or you can have my copy for $24...
... and I guess I lied. My review is longer than the game...