Recently I bought this game through Steam to play it in co-op with Sgaduuw.
And I must admit it’s brilliant 
Finally a game definitely worth doing in to co-op, though voice communication and a person you know are required.
It’s always fun to here someone at the moment they know they will die ’cause you stopped pressing a button, or placed you portal somewhere else.
The puzzle so far are doable, some take a bit longer, although I hope they will become a bit harder.
