Saw a lot of people who are into Metal Gear but not necessarily open world games saying they are dropping this game last night, and I would recommend sticking with it.
Despite actually liking open world games, MGS5 didn't really click with me until mission 12 and getting D-Dog. Up until that point I felt like I was still learning AI behavior and getting fully accusotmed to what works and doesn't, and my biggest shortcoming (not being fully aware of my surroundings, often leading to a sole guard on patrol finding me and shouting to all his friends) was now covered by D-Dog. Up until this point the game was fun, but now I got what everyone was talking about. Using decoys to make a soldier stationary while I circled behind, doing a double take down with D-Dog, hiding in plain sight and taking down the guard who made reaching a mission objective impossible, all so much fun.
So yeah, stick with it. The core trappings of Metal Gear are there, it just can take some getting used to all the extra stuff.
(RIP HAYTER.)