This game has loads of content and so much to do that you could still be playing through new material multiple years after you buy it.  With some help, I beat all the campaigns on normal.  Then I tried them on higher difficulties.  Then I tried campaign achievements.  Then scavenge.  Scavenge achievements.  Versus.  Realism.  More achievements.  Chainsaw massacre.  And yes there is so much more.  Every mode is a new experience that hooks you, as L4D2 is a bigger and better version of the original.  The original game's main draw was that it was a new, innovative concept.  L4D2's draw is the quality Valve has instilled into that concept.  More guns, new characters, and better hordes all make for a more complete experience.

Online, the lobbies are a bit slow, but once in a game, it gets intense.  Whether you are trying to hinder a roving group of survivors or trying to collect gas cans, the tension on both sides is huge.  Online play also often helps you develop your skills or earn achievements from the people you play with, as I experienced while trying to rescue the gnome.  Working together on those kind of group achievements is one of the best experiences in the game.  Overall, this is not a game to missed.