and I don't regret it at all. The surprise for me is ACIV, I am so into this game, it's absolutely huge (think Skyrim huge) and I am on a personal quest to complete every single corner of the game.
it's a great game that I would highly recommend, but I don't think I'd say it is Skyrim huge. I'm 30 hours in and other than some random chests/Animus doohickeys/bottles I don't have anything to do other than story missions- no assassinations, no naval contracts, all the Templar deals have been done. I think I did 3 full sequences before anything new opened up, and even those segments are fairly quick. Heck, probably an hour of that time is stopping at a small island, swimming to it, getting the items off of it, and then swimming back, over and over.
In comparison, I have 195 hours into Skyrim from 2 playthroughs and there is still stuff to do that I haven't seen in either my 80 hour first run or my 115 hour second run. You could argue that fast travel has cut down on play time for me in AC4, but I also used it just as much in Skyrim.