Well...it was quite an experience. As the final Metal Gear game by Kojima, there's definitely a lot that needs to be said.
Firstly, story-wise, while the game isn't filled with super long cutscenes like the older MGS games, there were still a lot here, especially toward the end. The length is fine for what the game is trying to be, but I'll admit that as someone who got interested because of the super long cutscenes, it is disappointing to see them slip away.
Really, the problem with the story...is the pacing. First you get a wild and crazy opening, then the game starts to slow down, natural for even MGS games I think. But then the game stays at a slow pace, with a few pick ups in speed, but usually dipping back down before it can leave an impact. The parasite craziness, while hinted at, can be easily missed if you're not paying attention, and then it'll come out of nowhere on you.
The biggest example is Eli. I can't recall once in my playthrough hearing about the White Mamba before his mission showed up, and then after you beat Eli, they just kind of drop him. He gets a cutscene and a rematch fight, but neither are really important. It's only when he appears on the chopper apparently working with Tretij that he becomes important.
That said, all the characters here are alright, except for two. Snake, of course, is horribly handled. This "Mad Max" approach isn't actually out of character for him, since he was progressively becoming more quiet, but it's a bit overboard here, and it's especially noteworthy during the jeep ride. Skull Face did ALL the talking, and Snake just let him rant. Respond, dang it, don't just let him think he's right. Make him question his actions, make him debate his viewpoint. I know Sins of the Father is a nice song Snake, but we can listen to it later.
The other character...is Miller. His "phantom pain" and "lust for revenge" (https://www.youtube.com/watch?v=7-TBGJ0yo30) is understandable, even if it's clouding his judgment. But because of that, he's made some strange decisions. He desperately wants Huey alive, even though he may be working for Cipher, but Quiet he wants dead because she mightbe a Cipher assassin? Quiet has a zillion opportunities to kill Snake but never does, but he still holds a grudge against her. But Huey? Not only is it 90% likely he betrayed MSF plus 10% for luck, but he's also proven to be withholding information. The only one who kind of calls him on it is Ocelot and some soldiers, but he doesn't really push too hard on the issue.
Overall, story-wise and character-wise, there's some interesting ideas here and really cool executions of those ideas, but others are just wanting, desperate to really shine, but held back by filler missions and other plots that also want to shine. Chapter 2 has the possibility to let that happen, but I have doubts, considering I know that A: some of the later missions are just hard mode of other missions; and B: Chapter 2 is only 19 episodes.
As for technical things, I like the music, even if we're missing a lot of good old Metal Gear tracks (is Yell Dead Cell here? Doesn't seem so). The graphics are fantastic, and the gameplay works really well. I hated the mission structure and base management in Peace Walker, but here it's much better. The Ground Zeroes data transfer was under-utilized, but oh well. I wish they explained the jargon a bit better (I still don't fully understand what archaea and wolbachia are), linked to Metal Gear better, but it's fine for what it is.
I'm not sure how to feel about Chapter 2. I assume it's going to resolve Eli and Tretij's plot, maybe some stuff with Codetalker and Quiet, and definitely something with Ishmael. I have a feeling it won't explain what's with Miller's eyes or the Skulls, but I guess they're not important. Either way, I'm enjoying the game, and hopefully it'll continue to be awesome despite any flaws it has.