9.7/10 is not for no reason a mark I give to this game! It is up there with my top 4 games, together with Super Hexagon, Kerbal Space Program and Euro Truck Simulator 2.
To the game, the game is stunning in so many ways:
Graphics: On max settings, the graphics are beyond beautiful and able to bring my GPU (GeForce GTX 460) on its knees. The water looks beautiful beyond your wildest imagination. The mountains look amazing if you look at them from a distance, and up close. They're just as good. Rico has some nice detailing when going through various biomes. The cutscenes are done in some weird style, which I don't really like, but it's okay.
Music and Sound: The music is one of the best music I've ever heard in games, ever. The anthem is just hilarious, the other music that randomly (or when enemies are angry) plays, is very good, and fits in really well with the game. The sound effects are nice. The vehicles sound like real vehicles. The grappling hook as that sound that makes it sound like you're really firing one.
Gameplay: The gameplay is amazing, it has nice controls. The only complaint I have here is that the default button for handbraking in a car/motor/truck is X, which isn't really easy to access on a keyboard, but you can change that, so that's ok.
Level/World design: The world is extremely detailed, and bigger than any other world (except for maybe skyrim) I've seen before. It's really diverse, you have deserts, mountains, jungles, cities, airports, everything.
Summary: This game is certainly a game you should buy, it's cheap, it's great!