Excellent game. This is probably my favorite game of the year. It's been nominated for GOTY on a lot of big lists, and I'm not surprised.
I'm not much for co-op games. So I had no intention of picking up L4D. But the NPC AI is amazing. Your partners will clear a room, point out needed supplies, heal you when you're dying, save you from a boss, give helpful tips and everything else you need them to do. It's like you're playing with a real team. I've never seen AI like this in a game before. So even if you're a single-player gamer, this is still a great game.
Also check out the guide book. They did a really good job. Even though the game is always unique, the book provides some backstory and lots of great tips. You can see a free preview here:
http://www.primagames.com/catalog/promo_image/9780761556985_1181.pdf
Anyways, feel free to hit me up at "TBDave" on the 360. Right now I'm playing this and COD5.