Great game. Buy it. Buy it now. Looks absolutely gorgeous on the Xbox One, but I'm sure it's nice on other platforms too.
They've addressed my primary complaint of the previous games using a startlingly simple strategy: by NOT adding a bunch of random crap, and instead dumping all that effort into primary gameplay. Sure, there's the modern world stuff with a few silly hacking games (though kudos to whomever snuck in prime factorization), but it's at best a brief distraction. There's days and days worth of looting, brawling, pirating, and assassinations to be had throughout the game. There are still side activities in game (eg. whaling, diving), but they are sufficiently tied to the core game mechanics that it doesn't break immersion (eg. spinner puzzles).
When they say that ship-to-ship combat is a core mechanic, they are not kidding. Sailing is very much an equal partner to the classic assassin gameplay, and there many sea missions in the core storyline. It's not quite as polished as the Assassin play, but satisfyingly fun.
The game is LONG! 22 hours in and it feels like I have quite a ways to go. It's hard to compare the main storylines because Black Flag draws you in so deeply into side activities. To some extent, it's required - ship missions will start getting unreasonably hard unless you upgrade your ship. Upgrades means materials and money and blueprints, which means going pirating on the high seas. But also, because there are so many small islands to visit, you can reasonably "complete" many zones in a short period of time, and thus are more motivated to do so. It was harder to get excited about finding animus fragment 112 of 250 in Rome.
The difficulty curve seems well-settled now. Particularly in open combat, you can't just counterstrike your way through fifty guards anymore, with riflemen sniping and brutes decking you with axes, and soldiers not lining up single file to die. Guns are powerful but fair. Ship combat is actually a bit hard sometimes, but this is more a symptom of the lack of a quick way to look around.
Finally, ways to die..