The Good
The "Golden Age of Piracy" Caribbean setting is really a lot of fun. The Assassin's Creed games have always had a good time playing with real history versus the added secret history of the Assassin vs. Templar conflict, and this is no exception.
The mechanics of the game are still very good, though there are still some instances where I think I'm having Edward Kenway leap in one (safe) direction and end up commanding him to fall to his (unsafe) death.
Sidebar: It's funny how sensitive I am to the cues the game is giving me - I flinch whenever I send Edward sailing into the air and realise he's not stretching out into his leap-of-faith pose, knowing that he's going to have a hard landing. I tend to get a tiny touch of vertigo whenever a character I'm playing falls from a great height, unless it's in a situation or a game where I know it's safe. An Assassin's Creed protagonist dropping into a haystack or the water doesn't make my stomach lurch; likewise I know for a fact that even if I let Batman in any Arkham game fall from the highest point possible, he'll slow his fall in the last few metres to land safely.
I have seen several comments, from both fellow players and reviewers, that they were annoyed by how much the game really was an entry in the Assassin's Creed series as opposed to more of a standalone pirate game just called Black Flag. I'm absolutely in the tank for the Assassins vs. Templars conflict, though, so while I appreciate the distinction of having a protagonist who's not actually aligned with either side but just interested in becoming a successful and wealthy pirate, I'm really satisfied with the extent to which the whole conflict and the Precursor civilisation still matters to this game.
On that note, I'm also probably one of the few people who was interested in completing the modern-day portions of the game, where you play an employee of Abstergo Entertainment who is responsible for getting the "footage" from Edward's life that they're planning to turn into an immersive virtual experience called Devils of the Caribbean. To be clear: I was irritated by the Frogger-style hacking puzzle, but I actually found all of the artifacts from the early days of successful Animus technology really interesting. Plus, ironically, Desmond and crew are probably more interesting in the information and voice journal entries recovered by Abstergo than they ever were in the previous games.
The Indifferent
The hunting and trading subsystems are simpler and thus better than in Assassin's Creed III, where they were first introduced. It's actually really cool to be able to choose to capture a defeated ship and send it off to your personal fleet. The downside is that the economics of your fleet have nothing to do with your actions in the game, really; it's a relief to not have to flick through submenu after submenu to make medicines and cabinets in order to send them off in trading caravans as I did in Assassin's Creed III, but all of the goods you trade in Kenway's fleet are specific to the fleet minigame. I'm not looting tobacco or tea on the high seas as Kenway and then selling it on, I'm just acquiring it from winning fleet battles as far as I can tell.
The tombs from previous games, which I always enjoyed whether they were exploration/parkour challenges or enemy bases you needed to fight through, have been largely replaced by the diving bell sequences. I actually don't mind the mechanics of the wreck-diving areas - mastering three-dimensional swimming while avoiding poisonous sea urchins and making sure you don't run out of air is fun. What's not fun are the fucking sharks. So far there's only one wreck I have managed to fully explore without getting chomped to death, and that was not a fun experience at all. The sharks add unnecessary danger and too many areas are impossible for me to navigate while hiding from them.
The actual gameplay of the modern-era sequences kinda sucks. The engine makes walking around in first-person view awkward, and the characters are clearly not as well-designed as the pirate-era's. Plus, as mentioned before, Frogger. The other two hacking puzzles aren't that bad, but fuck Frogger.
The Disappointing
The sailing portion of the game is a huge part of what you're doing, so it's unfortunate that I don't really find it all that much fun. Combat is fun enough moment-to-moment, but the difficulty is completely screwed up. At the point where I finished the storyline, I had upgraded Edward's ship with just about everything I could afford, but I still find myself facing multiple ships at a time that show as too tough for me to handle individually, much less in pairs or with smaller buddies along for the ride. Meanwhile anything less than a man-o-war I can take down in under a minute. In fact, my cannons are so powerful that I have to deliberately aim too high to deal full damage if I wanted to capture one of the smaller ships, or else I sink them in one volley.
I don't find it enjoyable to have to stop playing through the main storyline in order to screw around attacking ship after ship in order to buff up my own. At one point, faced with two man-o-wars guarding a bay I needed to enter, I parked my own ship off the coast and took Edward for a two-kilometre swim past them just to get to the mission start point I was heading for. Not, I think, working as intended!
So far the only way I have managed to survive is by taking advantage of the weird-as-hell design decision that you can board an enemy ship you have disabled even in the middle of combat with other still-mobile ships, defeat the crew and capture it, and then use its resources to repair your ship, all in a frozen instant of time after which the regular fight will resume!
Conclusion
I recommend that people play this game if they're interested in being a cool pirate and if they don't mind a bit of a resource grind. At this stage, I can't say whether or not I'll bother to try to take control of every zone on the sea and craft all the gear I can and defeat the "legendary ships" in the corners of the map . . . it really depends on how motivated I am. I'm already shortening my time spent on the sea by making frequent use of the fast-travel system, which feels a bit weird when I could be hunting for jerks.
Sidebar: Fuck Assassin's Creed multiplayer. I haven't enjoyed it since they introduced it, and while I intend to give it a shot again eventually even the tutorial reminded me how annoying and awkward it feels.