Assassin's Creed II was an interesting game for me. I played through most of it over the course of maybe a week, and then took an extended break of over a year before I finished it. Initially, I felt that the game was weaker than its predecessor, but now I'd say it's on equal footing, perhaps better.
While I enjoyed the story, I can't say I was incredibly invested in it. That said, Ezio is a really great protagonist, and this game does a great job of establishing and developing his character. Ezio is what really sells the story of AC2 for me. The mission structure is decently varied, with a number of side missions you can also take part in. I didn't find much of the side content that interesting aside from the assassination missions, but they're not bad by any means.
Gameplay is rough around the edges in many ways, partially due to aging, but it's still a lot of fun. Parkour is the highlight, with several improvements over the first game and a nice freedom of movement. Combat on the other hand... well, it's not awful, but it's not great either, especially compared to the other two games in the Ezio Trilogy. You'll often find yourself slowly whittling down enemy health later in the game, which can make fights a bit tedious, though the mechanics themselves are pretty fun. I find that it becomes a chore against tougher enemies, but lower-level ones are still pretty easy to fight.
Finally, this game has a really great world. Like the first game, there are multiple cities to visit in AC2. Florence is my personal favorite, but the others are great as well. They all have a unique feel and personality to them, which helps set them apart from each other. They're fun to traverse, and one of the best parts of the game.
Overall, Assassin's Creed II really lives up to its reputation, I think. Some aspects haven't aged incredibly well, but if you're willing to look past that, you're in for a great experience.