The best game you will ever play.
The single-player is captivating, but short. But no worries, because there's an entire co-op campaign! Finished that as well? No worries! The new Perpetual Testing Iniative includes and new ridiculously easy to use puzzle maker, and map and mod sharing via steam so now you'll never be short of puzzles.
I struggle to think of bad things about this game, the story slowed down a bit in the middle, but soon picked up again, the the official campaigns are too short, giving about 12-15 hours of gameplay, but with the new update this wont be a problem. The campaign puzzles tend to be a bit to much on the easy side, but again the PeTI comes to the rescue allowing for as challenging maps as you want.
Fun all round, you'll be crazy to not buy it.