Short review about game itself, co-op, versus and singleplayer modes.
Versus is perfect if you want to enjoy having to fight human-controlled zombies, or if you want to be one by yourself. It's like Co-Op, just Special Infected are more smarter than Computer-Controlled.
Co-Op is better than Single-player, of course it depends what teammates are in your team. Co-Op is made so you can enjoy doing campaings with your friends, co-operate with each other, enjoy the game and other reasons that I haven't found out.
Single-player is worst game mode yet I played in, it's really boring. You have to play with bunch of robots who cannot get with you, get stuck somewhere, shoot your teammates etc. They're really annonying, you almost always have to go first in some dark areas, robots will only follow you after about 3 seconds, in those 3 seconds you can get pulled by smoker or get knocked out by rock flying from Tank.
So that would be short review about this game, enjoy it. This game is perfect for those who enjoy combat, as combat here is the main thing. By the way, don't forget - Versus and CO-Op REQUIRE teamwork!