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.
As far as cooperative horde-mode games go, this game is the apex of the FPS food chain. There's plenty of room for this game to improve balance-wise. However, over the years since the release of the game, a lot of content has been added to the game that made the experience even more richer.
As of the time of writing, the game has 61 guns to play with across 7 perk classes, 26 maps included where you can blast 10 different types of mindless mutants, each with their distinct characteristics. There are at least 5 difficulty levels to play on, Beginners for people who never played a shooter before, Normal for the greenhorns, Hard for the veterans, Suicidal for masters, and Hell on Earth for masochists.
If that is not enough to tantalize you, there is a massive library of at least 100 extra creative maps from the modding community that can be readily downloaded from servers that run them, and plenty of mods that add extra monster types, perk classes and weapons. Go get it, now!
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.