There is absolutely no way this is worth 20 euros.
This is a glorified prologue for Metal Gear 5. Something that should have been included n that game – or be a free DLC at this point, which it... is? Because not only is it practically the same game, Ground Zeroes give you less to do than Metal Gear 5. So it is just a worse version of that no matter how you look at it.

All that being said, if you can get it for free, or maybe for a few euros, it actually is quite cool. As a fan of Metal Gear 5 (well, I have mixed feelings about it), Ground Zeroes managed to keep me entertained for almost four hours.
So if you want to spend four hours lollygagging on a new map and are willing to pay a few euros on this (meaning you need an 80-90% discount), then maybe give it a spin.
Or, as some other reviews state, you can probably get it if you do buy MGV (I don't remember if that is how it ended up in my library)

But I simply can’t recommend anyone to spend full price for this thing.

I am Big Boss and you are too.

I remember getting this for like 3 bucks once but you know what? When the game came out it was like 60 bucks...

60 Bucks for a demo? Like for the main mission you can get it done in like 1-3 hours or so, the rest is just secundary missions.

Its a good way to showcase what was to be added to the real game but i don't see it even worth buying at full price at 20 bucks like how it is at the time of this review.