I was one of the people who loved the first Assassin's Creed. That game took a lot of crap for being too repetitive (which was a fair complaint) and for having a somewhat obtuse storyline and a not-super-likable main character (which didn't bother me as much), but I really enjoyed the repetitive things I was doing, so I didn't mind being asked to do them over and over.
The free-running and environment climbing was superbly done, and the combat was the most cinematic I'd seen in a game, bar none. I would get into fights just to watch the fantastic counter animations play out - and to feel like a badass, a feeling the game provided readily.
Assassin's Creed II appears to trump the first game in pretty much every possible way. The video below is lengthy, but if you have a half hour and any interest in the game, I recommend watching it. The guys over at Co-Op always put together a good show (you should go visit them over at Area5.tv, and subscribe in iTunes if you do that sort of thing), and this one is entirely focused on Ubisoft's newest entry in what will now, I'm sure, become a headline franchise for them.
So why am I not playing it? Largely because I'm stubborn. I have an XBox 360, but no HDTV yet. I played Assassin's Creed I on the PC, and it looked fantastic at 1280x1024. I want to play ACII the same way, but the PC version doesn't hit until March of 2010. March! So I'll probably bite the bullet and pick up the 360 version, but I haven't yet. In the meantime, I'll watch this video again. You should, too.