There's not much to say about this game that probably hasn't been said. It's a good game, and if you're fairly new to the series, it probably feels like a great game. It looks stunning. It's got a pretty cool story. It's challenging, yet rewarding.
However, if like me, you've played all of ACIV's predecessors, it feels pretty redundant. Run over there, kill those guys there, don't let those guys see you there, run back over here; great job! Now do it again!
ACIV is yet another game in the series that bets its success on the fact that franchise players still want to see how this never-ending story wraps up. Yet, it suffers from pretty much all the same problems that have plagued the series in the past. Tedious economy and side missions, janky controls that send you running up a wall or jumping off a rooftop in a direction you never intended to, and messy tutorials that leave you guessing has to how to use your skills effectively. There were also random bugs in my play through that caused me to be unable to finish certain missions without completely restarting the memory.
I guess what's the worst for me in every AC game is that the missions outside of the main story line get extremely repetitive and often tedious. Underwater missions were nearly impossible at times. Ship battles start out as very exciting, then after having to do them over, and over, and over...and over again you start wishing you could just get from point A to point B in under an hour and a half (yes, I'm aware you can fast travel, but seriously, for a game that's about being a pirate, the open sea is a shit place to be). And the trade/commerce battles are just awful. Not to mention, if you want any sort of relevant cash flow, the trade/commerce option is the only way to go, and you can end up spending multiple hours just dealing with your trade routes.
All that griping aside, there are a lot of fun and interesting moments. Some pretty big questions get answered throughout the storyline, and it's really fun getting acquainted with a new ancestor who himself is becoming acquainted with what it really means to be an assassin. If you really love what AC games bring to the table, there's no doubt you'll enjoy this installment in all it's seafaring splendor. Just don't be a completionist nerd like me; play the story, and have fun.