I pre-ordered L4D on Steam and played it A LOT the first month it was out. Then I took a few weeks off, played through Fable 2 and finished off Fallout 3, started Dead Space, then this past weekend, I loaded up L4D again and started playing. I don't know if anyone's told you, but L4D is awesome!
It's probably the most fun you can have just picking up a game with 3 strangers and seeing what happens. It's alright playing with friends, but after awhile, you know exactly what they're going to do and how they're going to handle any given situation, but with strangers, anything can happen, and it's usually more fun to lose than to win. People in general dunno wtf they're doing, not that I do, but some people, yeah, they REALLY don't know.
This weekend, 2 of us got whacked on expert and me and this other guy were going through, and he spooked the witch, so I just stood there and didn't do anything cause I was a 1 HP and seeing B&W. So the witch killed him and ran off, and when I couldn't hear her anymore, I start moving again, opened a door, and right outside the door a Boomer head butted me or clawed me or something, and I just died. It was hilarious, all that work playing it safe, and a Boomer kills me without even trying.
That's the kind of thing I love about L4D, it's never predictable, and anyone who says it is sure as hell isn't playing on expert!