You should check it out. While I do consider myself a fan of the AC franchise in general, I wouldn't say I'm a rabid fan of it. But Black Flag is just fucking fun as hell. It's a huge shift in the game focus, as far as gameplay goes. Yes, the PLOT of the main story, mostly involves being on land doing climby stuff, the main game loop is about the ship and sailing. And it's incredibly engaging in my opinion. From the ambient sea noises as you crash through the waves, the singing of your crew (the sea shantys, my god the sea shantys are awesome and provide huge atmosphere to the sailing), the weather coming up and making things dangerous. Just, all of it is really damn fun. You will EASILY find yourself ignoring the main story of the game (which is easy to do in all of the AC games, given how much side content there usually is), and just let yourself be a pirate. The ship combat is fun and engaging, and it's really awesome to do a boarding action and go leaping from the rigging of your ship to the enemy ship, then leap down on them and start cutting away. The ranged ship combat is fun, and you have to account for waves and angle and momentum, it's just, really damn fun.
Highly recommend it to anyone that wants to play a fun pirate ship/sailing style game, regardless of the AC skin to it.