Better than the original, which is saying something.
AC 2 brings a lot to the table, some of it old, much of it new. For the most part, this game improves upon the original in just about every way, so if you liked the first one, you'll probably love this one. AC 2 brings much need variety into the game with all sorts of new mission types, and they are most welcome. Plenty of new sights, weapons, modifications to the health and items system, and much more customization in general. The new features are tasteful as well, and don't seem like the designers just tried to force a ton of new content on us in the hopes that something would stick.
The graphics are certainly good, but did not feel like a substantial improvement over the first game to me. For the most part, that's a compliment though, as the first game didn't leave a lot of room for improvement graphically.
The Sounds and Music feel vastly improved in AC2. The music really sets the tone well, and the effects are spot on. If nothing else its great to be able to play without the annoying cries for help in the form of AC1's ever present "Save the Citizen" missions.
AC 2 is up there with the best of them in terms of story telling, and this time not just in the games future setting. They really do a good job of developed the protagonist, Ezio, throughout the course of the game. The overall story line is well thought out and delivered. It also doesn't make you want to smash the buttons to skip through every last cut scene. Fortunately the speeches your enemies make after you assassinate them are much shorter this time around too.
The game is not without flaws though. Despite the vast improvements, there are some things you'll actually lose from the first game. For example, even after beating the game, you are unable to revert back to previous points in the game to replay key sequences or movies. This is disappointing since you'll want another go at many of the games highlights. Unfortunately if you beat the game, watch the ending movie, and are scratching your head wondering what just happened in that movie, your only choices are YouTube, or going back and beating the whole game over again and paying better attention next time.
Also, the climbing and platforming seemed to take a step back for me in the second game. I don't know if its because I've played other games like inFamous with unbelievably fluid jumping and climbing mechanics since my last go at AC, or because of differences between the PS3 and 360, or because they intentionally changed it or what. For some reason though, the climbing in the this game is much more frustrating than the first. I committed 10x as many blunders in this one than in the first, and it did dilute the experience just a tiny bit.
The game has only been out for a few days, so it's tough to say what the replay value will be like. It certainly seems to have more than the first, but that wasn't necessarily one of AC1's strong points to me.
Overall, AC2 is highly recommended despite a few small flaws. I feel it is worthy of mention among the more elite games in recent history. Most importantly, it feels like it was designed by a team that listened to what the fans and critics of the first title had to say. I sincerely hope that is a trend in games that continues.