Left 4 Dead

Was looking at getting this before christmas. People on Xbox live seem to think its amazing, and the shops seem to be sold out or, like in Zavvi, charging £44.99 for it. Haven't played it. I understand its a Zombie game, but what exactly is so good about it?
 
It's a co-op game that really forces you to co-operate as a team, especially in the higher difficulty settings.

If you must, download the game on your PC and get garena first to decide if you should buy the game. I did.