I've only played the first game as it didn't get me interested enough to bother with the second one and here's why ...
The game is definitely well made, especially the architecture of the cities and the parcour mechanic that uses said architecture to great effect. The combat was relatively engaging, for a while. The story was creative, kinda.
The problem? It was repetative as hell. All the mechanics got completely old in the first missions and were used the same way in every mission afterwards(except the ending that did away with the parcour). The formula for literally every mission was: go to the bureau, get some info, kill the target, run away, back to the bureau, back to the base, rinse and repeat. Nine times.
The parcour was fun and well designed but that too got boring once I had visited every city and seen the sights and every move Altair could do. Which was less than a third through the game.
Same goes for the combat. Interesting for a bit but, then again, most new things are. Once you get used to it and the novelty wears off it becomes quite repetative and boring. The developers tried to keep it fresh by giving upgrades from time to time but they were so minor that, atleast for me, it didn't really help
The story may have felt more engaging if it wasn't delivered in too many identical parts. A dead guy tells you some cryptic stuff and your character seems too dense to understand any of it. In the end there's the great twist that I personally found to be somewhat silly.
What I think the game was lacking was more elaborate setups for the assassinations. Every time it was just: "Go get into the guy's place, walk up, chase him if he runs and kill him. Then run like hell." It would have been far more interesting if the developers had taken hints from other assasination games (by that I mostly mean the Hitman series) and made the assasinations (at least potentially) more elaborate where you could set it all up so that no one sees you coming or going and you're long gone when the target drops dead or the corpse is found. Here it was just, mostly, stabbing the target with a knife at point blank range and getting chased by guards (all of whom can, apparently, perform le parcour to some degree) afterwards.
Overall, I'd rate the game 6/10