I loved the first game, and I loved ACII even more, in fact Assassin's Creed II was my GOTY (And if anyone mentions Uncharted 2, yes that game was awesome but I still liked ACII better.) of last year. In my opinion, even if you didn't like AC1, its worth playing. It improves on #1 ten fold, while retaining the elements that made #1 good. The controls are smooth, the graphics are breath taking (Watching the sun rise on the crystal clear waters of Venezia... WOW. Kinda makes me want to go back there again. Curse my empty wallet.) and the story is good, even if it gets confusing at the end, hell, even the character says "WTF."
Assassin's Creed II improves mostly in variety. The biggest negative of part 1 was its repetition, essentially doing things repeatedly with little variation. And while the story was great, and I have praised GTA IV's story despite it pulling this tactic (And me still not approving of it) but I don't like it when a story stops and before you can push it forward significantly you have to pick up tidbits in a random order. I also felt Altair was a weak character and didn't really have much of a past. ACII fixes both of these, there's a larger variety of side quests and they all have a different "Set piece" each time (I.E. rather than just repeating them with a different character model, they actually have objectives and different locales etc.) and they even have assassination missions, which helps the immersion factor since it was somewhat annoying in the first game you would take forever to actually.. well, I dunno, ASSASSINATE someone. They also have a linear plot path this time, and having a linear plot path keeps it fresh and easier to follow. That and Ezio is a much more interesting character as they have a plot behind his past (Although it was excessive to start right at his birth) and they build up to his reason to becoming an assassin, and while its fairly cliched, it works and it does provide a more logical entry point.
That's another thing about ACII's plot I liked better. In AC1, Altair was already an Assassin and you weren't learning how to become an Assassin so much as you were learning how to clean up a mess as an Assassin. ACII actually has you becoming an Assassin and going through their ways etc. etc. etc. and it felt more rewarding in that regard. I will say the ending of the game is confusing, again, and I can also say I wish they had more sequences with the flying machine than just the training sesh and the one mission. Regardless, having Da Vinci was awesome and a history Nerdgasm, and there are other historical figures that pop up.
My biggest negative for the game is that getting money is relatively easy, after a certain point your villa will bring in enough dough that you could probably put a price on the Ark of the Covenant. This makes armour, bribes, and weapons ridiculously easy to collect, and the more you collect the more money you actually get. They pay you to buy stuff. I'll also say that "The Truth" didn't hold much Truth for me. I kinda guessed who the two figures were instantly (Though I still won't spoil it) but honestly I didn't get what "The Truth" was and I think if you are going to have a set of tough puzzles searching for answers, they should actually give you more answers rather than another question.
I have the PS3 version, and honestly I feel its the weaker version. Mainly because the PS3's graphics have proved to be weaker with multi-platform games, and not only is there annoying pop-in there's also an unsteady framerate, I've seen the game drop to 18 on the PS3 version whereas my brother claims the 360 version he has never stutters. Also the PS3 version is glitchy, and has been known to crash and give you "The white screen of death" which prevents you from progressing. I had this for about a week and a half and it ticked me off.
This is a nitpick, but as a history dork there was one bit that REALLY ticked me off. After you beat the game, you can go back to a family tomb and there are chronicles of one of your ancestors becoming an Assassin. The problem? He was trained by Dante Alighieri. Does anyone see the error in that? If the Assassins are fighting the Templars...... How the hell could Dante Alighieri be a GOTDAMN ASSASSIN? Did they actually do their research on him? Ahem, the whole thing about him being a Templar? Whatever.
All in all: Its an awesome game.