The game itself is basically a co-op puzzle platformer (you can play it alone, but its designed to be played with friends). You and a friend have to work together to get through dozens of levels that are full of razer blades, lazer beams, and cats. It's a lot of fun because you can mess with your friend as you play. You can shoot him as he tries to time that difficult jump perfectly, or jump onto his head, or throw him into a pit of acid. It's silly, but the mechanics of the game are actually incredibly well designed. It's sort of like Super Meat Boy except more forgiving and you have to play with a friend for maximum enjoyment.
Here's the part where I usually talk about how great the story is, except in this game its nothing special. It's silly and weird, which of course fits the bill, but that's not really where the value is in it. The best part about the story is the narrator. During all the cutscenes he is narrating what is happening with the story as he uses puppets to also describe what is happening, and pretty much everything the narrator says is hilarious. He also says things while you're playing the game. For example, one of my favorites is this quote, which is prompted when you start a time trial: "Can you get the best time? I can, and have. A billion times over! I'm also a pathological liar. Or am I? I am. What's going on? Do you know? I don't. What?"
Basically, every aspect of this game fits together like a puzzle. Every game has a distinct 'feel' to it, and this game captures it and punches you in the face with it over and over until you've lost all your teeth. You pretty much know what to expect after watching the trailer (which you should definitely go see). The visual aspects of this game are all silly but they make their own sort of sense. The story is silly, but its cohesive. The narrator is silly, but that's basically his entire character.
I, personally, have only really experienced the story mode of this game. It takes longer to get through than you would expect (There's nearly one hundred different levels, which is insane to me). A lot of what I've seen in media, however, is the 'arena' mode. This is where you and three other people play on maps to achieve goals against one another. I haven't played it, so I can't say much about it, but it's probably still a lot of fun.
So if this game sounds interesting, I recommend going and checking out the intro about Hatty Hattington and the S.S. Friend Ship. It's about three minutes long and as I said it describes what the game is going to be about quite well.