I'm huge MGS fan. I have played all of them since the first one hit PS1. I have greatly enjoyed all of them, so my disappointment might be a little pretentious but hear me out.
Every time when MGS game came out some breakthrough took place: graphics, gameplay mechanics, storytelling, best cutscenes in the industry... With Phantom Pain was exactly the same: game that weight about 28 Gb, looks better than any other, with breath taking cutscenes, amazing open world and basically everything you would expect from open world game. But it only last for about 10h. After that there's 100h of repetitive gameplay with maybe 2 fun missions.
First thing: this game is not about the Snake, it's about Mother Base and Otacon. It's strange since this is MGS where Snake/Big Boss is the main character. Kojima is trying to create backstory that would make sens but after all you left out confused. Phantom Pain have one of the worst story in MGS series. MGSV is much alike any other snadbox games: huge world to explore, lots of stuff to collect, huge arsenal of guns to try AND story/characters that are totally irrelevant. This game is a lot like most Assassins Creed huge world, nice views, stuff to "do", but no meaning to it.
Other thing that even players not familiar with MGS would be rather disappointed about is that "new" mission that appears later in the game are exactly the same mission but with new requirements (very high difficulty, no items, total stealth etc) with exactly the same cutscenes, the same intro and you cannot progress without finishing them. All the same except for the mission/episode number. Why? No clue. No explanation. What's worst there's not 2 or 3 missions but like 25 of them!!! So half of all episodes you just replaying old missions. This system seams like it confused even programmers cause game have huge problem recognizing your progress and a lot of players (including me) have problems progressing as "new" missions do not appear due to the glitches (you can encounter glitch if you used wrong Body, Item, you did not listen to the tape etc...). For series that was known for its story this is just stupid.
To MGS fans: this is Peace Walker with Snake Eater but completely bereft of the story, characters, meaning. 95% of all of the story you get from tape recordings you may listen whenever you want. It's a cheap way of storytelling, and if you happened to not listen to it you may end up not knowing what is going on. It's little like Dark Souls with its items description. I think that most MGS fans will be greatly disappointed. This MGS should be called Snake Walker or Peace Eater: then the name that makes no sens would fit well to the story and the rest of games elements.
Online component is fun for few hours. Good for fans, disappointing for players. Invading other players Mother Base could be fun but it also grow old after few times. There's one rule: If you have a Nuke you are the target. I you don't you may never be invaded.
This re-doing of mission in-between of story-related missions is really disturbing. It's unnatural, makes no sens in terms of the story and it's boring in most cases... This is even more "painful" as the story is rather scares (not only for MGS standards but generally) and when you are expecting more of it you are getting old missions with new difficulty. Then the ending comes in and you are like "Why? Why would you make me play another 40h if you could just end it so long ago?"
Technically this game is definitely good... But it will not make it up for time wasted on stupid missions.
Give it a try but be do not get over-hyped. There's very little story, very poor characters, dull environment, and repetetive gameplay.