I’m playing this right now, and I can tell you that it’s pretty tight. Per this IGN report, they’ve added a few new mission types (I wouldn’t know since I didn’t play the 360 version), tweaked the AI a bit, and they say that it actually runs faster in DirectX 10, which was certainly not the case with, well, any other game I’ve ever played. I can believe it actually, because it actually runs quite well on my rig, and buddy, it looks fantastic. You can read the story at IGN if you want, but there’s not a lot more than what I’ve just told you.
Assassin’s Creed PC Q&A [IGN PC UK]