So far, the Left 4 Dead experience has left me unsatisfied. Single player works. But that's not really the point.
I managed to connect to one fellow Penny Arcade forum goers game, but even with a 100ms second ping (which is normally an okay ping), the zombies teleported all over the place as I was trying to shoot them. This lead to friendly fire and general confusion. And it crashed trying to load the second level. I tried to get into other people's games, but they were all reporting IP addresses from the far side of a router (192.168...), so they were unjoinable. Trying to start my own game, I was told that there are no dedidcated servers and I would have to host a LAN game. I'm amazed the other humans were able to move around at all. Or maybe they were instantly replaced with bots, and I didn't notice. Regardless, the netcode on PC seems pretty boned.
I thought this was a demo. It feels more like a beta. And with two weeks to the game's official release, that's cause for concern.