I thought it was aight. The whole nanomachine thing was too much of a cop out to wrap up loose ends. In addition it retroactively degraded the mysticism in previous entries by explaining otherworldly concepts with nanomachines.
Thematically speaking it lacked the nature vs nurture theme that was prevalent in the first 2 entries. I personally was a huge fan of this, and loved how MGS2 just went head first into it by ultimately providing a meta critique on the player. Some may argue that this was just a giant clusterfuck, and kojima's message was lost in being overly ambitious, but I always appreciate an artists's strive to push the limits at the expensive of a less cohesive output. At the other end, MGS4 was very streamlined in its plot in attempting to wrap up loose ends and ambiguity as direct as possible. While this is great for a cinematic experience, it felt a bit stale to me.
Anyways, I thought it was a decent finale. Themes of acceptance play well for closure, but I do wish it explored more metaphysical concepts as they tie into human individuality; but that might have been milked dry by the first 2 entries so who knows.
Gameplay speaking the hard bounce between acts made the game feel less connected. I always appreciate a good hub world, and this game really moved away from that. I mean MGS3 was already exploring that, but this made discrete areas that were explicit to the player.
In terms of characterization, I wasn't a fan of the Beauty and the Beast unit. They were cool in concept, but they were extremely forgettable. I honestly don't even remember if they had any dialogue that was beyond nonsensical blabber. MGS1-3's antagaonistic mercenary groups were very memorable and played a huge part in atmosphere so it was a bit disappointing that the Beauty and Beast unit were so lackluster.
Overall aight game. Kinda felt like Kojima was in a rush to tie up loose ends and didn't know how to do it well. But I'm glad he did it.
I last played it in 2010 but I'm about to replay it. At the time, I was putting it on my top games ever list. We'll see if it still is. So strange to this day 12 years later PS3 is still the only console that can play this gem. And strange to think MGSV is also on this same machine.