First of all let me say... this game is not worth $20. Did I personally get $20 of enjoyment out of it? Yes, but I'm a freak who got it at launch for more than that and still say yes. But you? Get it on sale or part of the MGSV bundle.

Moving on.

It says I have 10.6 hours in this game but I've also played the PS4 and Xbox One versions ad nauseam, so I've probably sank like 40-50 hours into it across all platforms.

First off I'll say if you're playing the Metal Gear Solid series out of any interest in the plot and lore, this game is a must.
It acts as a bridge between Peace Walker and MGSV, honestly it's really a prologue to MGSV which is why it's often included in every bundle and re release now.
It has not only story that ties the games together, but expands on certain plot elements and lore in the bonus material found in extra missions and collectibles. If you're doing a deep dive into lore, there are some tidbits from this game that are noteworthy. Also it's just essential because like I said it's literally the prologue to Phantom Pain.

Graphics wise, this game is gorgeous. The Fox engine does a fantastic job of pretty much everything you can imagine, and it does so as well in MGSV proper. It's a shame that after the original Kojipro was shuttered, Konami kept the license and now only uses the engine for their lackluster PES games and Pachinko cash ins. But anyway.
Sound design is good, nothing groundbreaking but not bad.
Voiceover is good aside from the replacement of David Hayter with Kiefer Sutherland, that still grinds me.
Music is good! Still done by Harry Gregson Williams like all the others. And the inclusion of the song "Here's to you" by Joan Baez is very fitting and a tremendous emotional tie in to MGS4 in my opinion.

Gameplay.
Missing a few features implemented in MGSV proper (such as the cardboard box, wall knocking, and weapon customization), this game is still the ultimate in tactical espionage action you know and love from the MGS series. MGSV and GZ have the best enemy AI of the series, tight gunplay, responsive controls, and are overall just a blast to play over and over.
The main mission of this game on average will probably take you 20-30 minutes your first go around. Yeah, it's short. It can be speedrun in as little as like 3 minutes.

But that's not the point. The real meat of this game is in replaying it. Camp Omega is a small yet wonderfully crafted zone to infiltrate, doing so over and over through different methods and routes, experimenting and goofing around the whole time you do.
There isn't just one mission either, there are several. They all take place in Camp Omega, but have different objectives, weather, and times of day. Yes it is somewhat of a tech demo showcasing the adaptive enemy AI and stealth systems, but it's also fun for the player to go through all these wildly differing scenarios and adapt themselves.
This is emphasized in that each mission also has varying difficulty levels, bonus objectives, collectibles, and scoreboards for you and your friends.

This game isn't meant to be a long narrative MGS experience, it is a MGS sandbox meant to be explored and replayed.

I've spent hours trying to beat my own mission times and scores, get longer range headshots or other high scores, and just playing around having fun.

The game also has a companion mobile app (same with Phantom Pain) which you can use to control your iDroid and also play a Mother Base building game. Doing so actually rewards you with some neat bonuses when you start MGSV. It doesn't work for the Definitive Edition of the game which has TPP and GZ bundled, so your mileage may vary.

Also completing special side missions in this earns you other extra stuff on starting MGSV.

If you're new to MGS, this and TPP are an excellent point to jump in on. However, if you really want to get invested in the story I'd say maybe try MGS1 or MGS3 first, but this still isn't the worst place to start. Gameplay wise though it is one of the best in the series.

Overall, this game is one of my favorite MGS games and even still sometimes I like to pop in and play a few missions to mess around. I give it a very good score. I rarely ever give 10/10 scores except in exceptional cases (like MGS4) so I will give this a 9/10. Which is actually a higher score than I'd give The Phantom Pain, because of the story. But I'll write another review on that later.

Anyway. I RECOMMEND THIS GAME!