After playing the Left 4 Dead demo for a good week I thought I could drop some knowledge on what I think about it for anyone that's still shifty on wanting to pre-order it for the demo.
The short answer is: Get it.
Forget about any other zombie games or mods you have played because this one blows them out of the water. There's an easy-to-use server search option that you and your friends can use to easily join up and be blasting zombies in a matter of minutes. The game obviously shines when you have 3 other friends you can play with but if not friend only games soon become public games so if it was just you and one friend eventually others can hop on to help which is a lot better than having the other 2 characters AI controlled, but not to say the AI people are bad, they are very good and stick close to the group so the game never feels like an escort mission.
The demo is rather short but the difficulty can be changed for some added challenge and the new AI "Director" ensures every time you play the level something new will happen at random so the players are always in for a fresh experience each time they play. Depending on how you and your friends play the AI will adapt and items found throughout the level such as pipebombs, molotov cocktails, better weapons and health pills will always change locations. The "boss" zombies add strategy and tests you and your friends ability to work together in stressful situations, at random a smoker zombie can latch its long tongue around you or your friend and you must shoot the smoker to release your friend or maybe a rather fat zombie called "bomber" will spit on one of your allies attracting a horde of zombies to come after that person forcing you to hole up quick and help defend your friend. Everything can happen at random and I can go on and on about different scenarios I've encountered.
Although the demo is short you can play it many times without being bored and this game surely will not disappoint when fully released on November 18th.