If you were disappointed with the 1st game, or don't know where to start with the series, allow ACII to make you a fan
in any case, ACII changed all of that (but to be fair, i haven't played AC3 yet...). it eases you in to the deep and satisfying combat system without overwhelming you and the story -- while a bit confusing if you haven't played AC1 --is still intriguing.
the graphics are gorgeous, the cities are feel full and alive and are a joy to explore. this is one of the best open world games to date.
my only complaints are that the economy feels a bit limited (though obviously an improvement over the first), and there are some visual quirks and AI glitches, particularly with your allies. last and probably most annoying is the fact that there is no difficulty setting. it'd be nice to be able to ramp up the enemy HP and AI, but as it is, you have to consciously make it more challenging in-game by avoiding buying new -- and cooler, of course -- weapons and armor, and that's just disappointing.
the bottom line though: this game sucks you in and sticks with you. the wonderful voice acting and cut scenes have you really sinking into ezio's shoes, and the gameplay is so addicting that randomly free-running around rooftops sneakily killing guards or taking one of the many side quests is as enjoyable as the main story, which is a good length in and of itself.