I don't normally write reviews, but as this is an indie game which I feel didn't get the recognition it absolutely deserves, I figured I should write something.
At its heart, UnMetal is, obviously, a parody of the Metal Gear series. This means that the basic gameplay loop consists in traversing spaces while avoiding and suppressing guards. The way progression works, however, can be compared to that of adventure games: you find an obstacle and you come up with a solution from your inventory, which often requires you to combine the objects that you have. In addition to this, the devs clearly had fun mixing up this formula with other concepts, often departing very much from the basic gameplay, but always in a fun and clever way.
Of course, being a parody, it's also very funny. The humour often stems from the fact that Jesse is an unreliable narrator, which often allows the jokes to weave into gameplay in clever, unexpected (and hilarious) ways. The general silliness however never gets out of hand, and the story is actually quite gripping.
Graphically, the game is absolutely gorgeous. The environments, the characters, the cutscenes always look wonderful, and the amount of animations (and their quality) is honestly breathtaking. Seriously, it's among the best examples of 2D art I've ever seen. It's especially crazy considering how small the dev team is.
The soundtrack is excellent, and sound effects are good as well. Unfortunately, the English voiceover isn't always as good as the rest of the components. As a side complaint, I should note that it looks like sometimes the devs forgot to implement Playstation button prompts and the game reverts to Xbox buttons.
All in all, UnMetal is a wonderful game. It's funny, it's a good stealth game, and it's a technical feat. Absolutely recommended.