It's a pretty damn fun game, but actually has quite a bit of challenge to it that keeps you coming back to it. There are 7 classes you choose from, and you attempt to eradicate specimens (Zed, kinda like zombies, pretty much zombies) on a map of either 4, 7, or 10 waves on one of 5 difficulties (though most people just play normal), before the final boss round, which involves fighting the Patriarch. Now, he's not your everyday boss: he can run faster than you, turn invisible, has a rocket launcher that can instantly kill you and a minigun that rips you to pieces fast on the same arm, and can melee extremely hard, and juggle you between attacks with ease. In 6 man games, if you're the last on alive, you're usually screwed on any difficulty above normal (and normal is pretty difficult most of the time, depending on your class/level). I highly recommend it to anybody; the Zed all make the L4D2 special infected look like babies, especially the Fleshpound.
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