No spoilers.
70 hours of gameplay and I've finished the game. I was "47% complete".
While the ending was sad, it was a little disappointing. There are 2 chapters in this game. The first chapter is what feels to me the real storyline. The second chapter is very short (Only like 10 missions, while chapter 1 has 30 something) and feels very cut to me. The end mission is damn near impossible, and while I did complete it, it gave me a reward of taking away my best friend. It is very emotional.
However the gameplay is very nice.
There is a Clash Of Clans type system in it, where you have a secondary base called an FOB, which you can raid real players and they can raid you. You can develop a lot of security equipment which helps A LOT, and recruit more staff to secure the FOB. You can build nukes, which prevent people from raiding you unless they have a nuke, or they have 150,000 "heroism".
There is an endgame, and it is still sandbox-ish and you can finish your side-ops missions and replay primary missions. While there isn't much to do after that, you can still participate in FOB missions and that's pretty fun.
Weapon development is a nice idea. However, it is very hard to achieve a higher RND (Weapon development) team. A higher RND team means more weapons and items you can develop.
While I Fulton extracted 80% of my enemies throughout the game, my RND level was 29. There are weapons scaling all the way to 60.
There was originally supposed to be 3 chapters. It was cut by Kojima for reasons, and all these reviews saying content/10, its more of a content/7. It does have a lot of gameplay, but after you beat the game, you're asking yourself, "Was that it"?
I give this game a 6.5/10. It is a little pricey though.
This was my Steam review.