I played the L4D demo for a bit today, and I have to say that I'm very impressed. The audio, the weapons, and the enemies all make up quite an atmosphere.

There were times where we'd be completely swarmed, and things would be very chaotic. One guy's being dragged away by a huge tongue, one guy is down, and my friend and I have half of our original health remaining. On top of that, they were counting on us to save them, even though we were separated by a wall of ravenous infected.

The infected are a real terror, and I have to admit, it was very different from fighting everyday human players in CS or TF2. I'd never faced anything like those zombies in a game before, so initially, I was shocked. 

The only problem I have with the game is how sometimes the servers can be really laggy. But hey, I've only been to three servers that are like that.

Also, I suggest not playing by yourself with a group of AI. When in single player mode, you are basically the leader, and the AI only follow you. That doesn't provide you with much choice on how to approach enemies and buildings or if you want to strategize in some other way.If you're playing with people from RL, well...you know what I mean. They'll make their own choices and contribute to the team even more. 

So basically, I have no doubt that the game will be great. I'm expecting at least an 8.0 or higher. Can't wait to get it on the 18th! :D