Depending on the type of gamer you are, you'll either love it and all who developed it or you'll be using it as a Frisbee. It's basically like being in a zombie movie with intense action and tension. Thing is, the game absolutely REQUIRES Co-Op play. Not the standard "Multiple players on a team who can go around doing their own thing while still killing the enemy." In this game, all the other players are actually necessary, because certain zombies have special attacks that will disable you until someone comes to help. Also, the hordes of zombies can quickly overwhelm you if you take them on by yourself. The Versus Mode can be fun too where you get to take the role of the special zombies themselves against other players.
One of the major selling points can also be one of it's biggest detractions. While the game can be enjoyed by yourself with bot assistants, it is more enjoyed with human players, and XBL (and to an extent Steam) has a high number of d*ck L4D players. I'd strongly suggest finding good players and friending them and work with private/friend matches.
This is a pretty good review of it (NSFW):
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