Difficulty:
If you have taken the hurdle and the gameplay has understood (took me longer, just say: 40+), you can play smoothly. The difficulty increases as the game progresses gently. In between are real tricky chambers there. But after that there's recovery chambers which reward you with quick sense of achievement.
Wit:
On max! There is much to discover and in some reactions I had to laugh out loud spontaneous. I'm just saying: potato battery. What it's all about, you have to ask to find out yourself. In one chamber, there is a radio. If you go closer, the music is louder it. Did not know what to do with the thing. Next time I would just take it a few times chambers long. After successful showdown in the end you so about 10 minutes really celebrated class. I had a tear of emotion in mind :-)
Graphics, sound:
Wow! I loved it ...
Co-op:
Split Screen, completely different chambers. A complete second game world that I just check with my daughter. Here, then shimmy the depicted two Robbies in cooperation with the Level. The two protagonists, I would visually inscribedable as "Laurel and Hardy in metal". For shooting, especially when the two communicating or just to wave :-)
Released from 12:
The game is entirely bloodless. But psychologically it is nevertheless not without. Man is the only person (in co-op two Robbies) in a gigantic fully automated industrial ruin. That is sometimes quite dark.
Conclusion:
This game gets a place of honor on my shelf. It's EACH EURO VALUE! Thank Valve!