Not much to write: if you have more than 2 friends on the planet Earth, and like shooting zombies, than L4D is for you.
So far, the servers are smooth like butter with no lag, and the adaptive AI is great, making re playability almost infinite. Speaking of infinite, the number of expansion packs and down loadable addons is also infinite! There 'are' only 4 "chapters" in the game (each with 5 levels), each chapter on Advanced mode with 4 human players takes anywhere from 1 to 2 1/2 hours. Though there aren't many stories here, they gave you a ton to do in them. Verses mode (play as zombie), multilayer co-op, and the boring 1 player (CPU controlled allies).
Oh, and the AI is actually really good, best in a while now. The zombies as well as the CPU controlled players are quite smart and won't make you mad... much. (Tell Bill to stop setting off the freggin' car alarms!
Take the 1 player action out of the rating, and even GameSpot can't deny a 9.5