Very fun, but repetitive.
Running on top of rooftops, climbing buildings and fighting enemies is a lot of fun, however the tasks that have to be done gets really repetitive after a while. Assassinating the main targets are all really interesting, but you have to do a few side quests before you can assassinate your targets. It makes sense that you have to learn about your target's whereabouts before you achieve your goal, it's just that there is a lack of variety in these side quests. There are roughly 5-6 different types of side quests to learning about your enemy for each target. There are 9 main targets which Altair must kill, so doing these same side quests each time you have a new target can make this game feel like a chore. It's a good thing that getting to these targets are fun, since you can easily climb buildings and objects, otherwise it's not even worth it.
Graphics:8.5/10
Textures, real-time shadows, blur effects and dozens of NPCs that can be seen on screen are really well done, the view from each view point is really breath-taking, especially with the use of atmospheric perspective which really captures a sense of a large city. Each location is large in scale and also littered with objects to climb onto. Altair is also very interactive with all of these objects, especially when he is climbing, he wont climb anything unless there is some sort of geometry to actually grab onto, which is a very nice little detail. There are a few bugs to take note, however. There are some noticeable "pop-ins" with NPC's, some graphical pop-in and there are just some absolutely ridiculous bugs that occur such as two Altairs appearing on screen or a horses' legs flying out of place. This game looks as if it could have used a few weeks of polish.
Sound: 7.5/10
The acting is good except for Altair, which is a real shame since he's the main character. It's strange how everyone in Altair's time has an accent except for Altair himself. It seems as if he has grown up in the late 19th century America. NPC dialog gets very repetitive. It appears as though each type character only has around 5 lines of dialog and you can't even skip some of the things people say when you save a civilian. Some of the sounds are well done, however there are some bugs in this area as well.
Animations: 9/10
The animations are very well done. The way Altair climbs buildings and jumps on objects are all very convincing. Combat is pretty flashy when you use counter kills.
Playability: 7.5/10
Running around, jumping buildings and climbing houses are all very simple to do, however there are some instances when Altair will jump onto areas you didn't want to go to. Combat is fun but the camera during combat could use some work. When you lock onto enemies the camera can find itself in hard to see areas and it gets a bit annoying. Although the counter-attack camera looks nice, i'd prefer to just leave the camera as it is when I counter, since the camera would sometimes find itself behind another wooden stand.
Replayability: 6.5/10
This is a mixed bag. If you want to do everything there is to do in Assassin's Creed, then your options are to go back and collect all the flags, get all the viewpoints or save all the civilians or even just get all of the achievements, however all of these are acquirable the first time through, so after that there is nothing else.
Story: 7.5/10
I wont spoil anything, but the story is interesting and after accomplishing the game you will find out that there is still a lot of story to tell. After the credits began to roll, I felt a lack of accomplishment, as if I needed to know much more than what was told. Just like how I felt at the end of Halo2.
Rent or Buy?: I'd have to say it is a rental, or if you can borrow it from someone. While this is definitely fun to play, it won't last after you beat the game. Many people have beaten this game within a week.