A cooperative survival horror experience set in a variety of nightmarish and surreal settings. Band together with your team-mates to defeat increasingly overwhelming hordes of freakish enemies and eventually - a horrifying boss.
Its crack. That's all I can say. What's not to love about a cooperative zombie genocide game, with a very good variety of weapons (everything from grenade launchers, to chainsaws, to automatic weapons), perks, and mutators? The game is very unforgiving though, and some of the perks are useless in harder difficulties. It's addictive, fun, hard, and you can have fun with 5 other people.
Great game, very mod-able, but it has it's issues like any true game does. Perhaps I don't get the full experience though, playing on a mid-low end computer, but even so. Amazing game though, and the amassment of difficulties makes it easy for anyone to pick up.