Valve could not have made killing zombies with friends any more enjoyable.
There are four survivors, of which you get to play as. The maps are based off of 'campaigns' and the game contains 5 campaigns. The maps are linear where you, and the other 3 survivors must reach the safe room located on the other side of the map. A single campaign will probably take you about an hour to complete, but once you do that you could enter the versus mode, where one team plays as infected, and the other as survivors. Or survivor mode where you must hold out against zombies or 'the infected' for as long as you can.
Overall Left 4 Dead is an excellent game that will keep you coming back. It has great re playability with its different game modes, and competing against another team for points in the versus mode just never seems to get old.
Shooter Fanatics will fall in love with this game at sight, others will enjoy the team aspect of all game modes, and others the competitive aspect of versus.