I've heard a lot of people complain about Assassin's Creed freezing. I haven't had that problem, however, there does appear to be a problem with the sound. During scenes where the characters are talking to each other it seems that there are delays in the dialog which then results in the characters talking over one another. The animation itself continues without a problem.
I have another ubisoft game that freezes all the time, Rainbow Six Las Vegas. Both of these games are brand new, 2-3 days old.