I didn't think Assassin's Creed 3 was great. It was good, certainly worth playing at some point, but if your other choices are Dishonored or Halo 4, I would go with either of those over this (particularly Dishonored if you were itching for this type of game).
Welcome to the Internet. A game (or movie, or anything else) can be almost universally acclaimed, but any discussion thread devoted to it always attracts those who were disappointed. Just look at the Bethesda forums devoted to Skyrim. You'd think it was the worst game ever made.
So far, I'm finding AC3 worth the purchase. And I'm not particularly an AC fanboy (I thought AC1 was boring and AC:R represented going to the well one too many times). But I like the ambition on display in AC3; the setting is fresh, the story is more interesting to me and seems less convoluted (so far) than previous installments, and graphically the game is a step up from previous iterations.
In short, I'd say you can open your copy. if you let internet whining dissuade you from art, you'd never watch or play anything.