Ground Zeroes is a prelude to Phantom Pain that should have been included with that game. It's not worth paying full price for, buy it on sale. If you're only interested in seeing the cutscenes that bridge the gap between Peace Walker and Phantom Pain, watch it on YouTube. There is maybe 15 minutes of actually important info here. I have 16 hours in this game.

Ground Zeroes is an almost example of a perfect vertical slice stealth game if you're willing to stick around after its first mission. Its single map means that the dev team was able to cram tonnes of interesting content that gives you a plethora of ways to approach the location. It's almost like a roguelike in the way that you're encouraged to do lots of run attempts, trying to improve each time. There are some fun rewards in the form of Easter Eggs that are brilliant fun, it does a good job of making Ground Zeroes feel frequently worth your time.