I really enjoyed the Ezio trilogy, and IV and Rogue, but have yet to play the two current gen games. Is it the worst game in the series? Is it notably worse than the other games in the series? If so, what went wrong, and if not, why is it so maligned?
Take Rogue, remove all the polish, cut back the naval elements, and turn the story into disconnected Revolutionary War missions (You're on the battlefield -- charge the British! Here's Paul Revere -- control his horse as he provides navigation!) and you've got AC3.
I do feel that AC3 is really bad, but Unity is possibly worse and Syndicate is only somewhat better. Wasn't a huge fan of IV's story and characters, either, though.
On a positive note, AC3 does have a lot of links with Rogue, so you might enjoy it from that perspective (on the other hand, you'll lose out on the early twist, as you already know Haytham).