Repetitive? Yes. Fun? Yes.
Weaknesses first. What gameplay there is that takes place in 2012 is extremely boring and doesn't seem needed at all until you're very close to the end (I'm taking into consideration this being the only Assassin's Creed you have played.) Basically all you do is listen to the two scientists that are holding you captive slowly reveal to you what their plan is and what is going on. I'll be honest, that part of the story is pretty lame. Both scientists are interesting, but the female scientist (Lucy) only because of her body frankly. Worst of all, the game rewards you for talking to Lucy as much as possible with achievement points, which is tedious and boring. The main gameplay takes place in 1191, and you'll always be happy to be back there over 2012. However, the gameplay is very, very repetitive. It's always the same: your master (Al Mualim) tells you where to go, you head over there, you get to lookout points and survey the area (press Y), eavesdrop on and steal from the townspeople (all to help you with your assassination of course), rescue certain citizens from oppressive soldiers, and complete little side missions for other assassins. After you do enough of these, you will have enough info to take down your target. After this, run away to the local Assassin's Bureau without a tail on you to complete the mission. Save and repeat.
Okay, now to the strengths. This game is absolutely beautiful to look at. The distinct architecture, your character's robes, and all the environments are just gorgeous. As repetitive as climbing up the lookout towers is, it's my favorite part of the gameplay. Some of them are insanely high, and when you perform the Leap of Faith down into a haystack, as fakey as it is, it's awesome every time. The controls in Assassin's Creed are fantastic and very easy to master. You can perform almost all of your amazing death defying stunts simply by holding up on the control stick and pressing down on the RT. It just never gets old for me to run on rooftops, jumping on one skinny beam to the next while onlooking citizens worry about your safety and question your sanity out loud. Great fun. Assassinating your targets is pretty exciting, and getting into big sword fights can be pretty fun too. If you die, the game isn't very punishing towards you, it just loads for a couple seconds and you start at the town gate. Your character (Altaïr) is pretty awesome overall, from his costume to his amazing abilities to his voice. If I could be more like one person in video games, I think it'd have to be Altaïr. The game has a nice soundtrack as well. It's nothing too spectacular, but it fits the game well and is easy on the ears.
There's no real guarantee that if you enjoy stealth games (I.E. the Metal Gear Solid and Splinter Cell series) that you'll enjoy this game as much as I did. For me, faults and all, it's one of my favorite games of all time. Overall, although I can't exactly recommend this game to everyone, I'd at least say to give it a try.