Viciously entertaining, extremely immersive, and as original as a zombie FPS can get.

User Rating: 9 | Left 4 Dead X360
I must say, this one blind-sided me. I always figured it would be fun because it's made by Valve, but I had my doubts. Four campaigns? Reusing the Source engine? Adaptive difficulty (bad experience with FarCry 1)? But, wow. This is hands down, one of the most entertaining multiplayer games out there, let alone the best cooperative experience. The four campaigns play out differently every single time and still manages to surprise me every few levels with something absurd... Like a hunter, a smoker, and a tank spawning inside my safe house on expert. Scared the bejesus out of me. The gameplay is just insanely satisfying. Super violent (great headshots), massive numbers of enemies (accompanied by a good framerate due to a clever corpse density system), and the idea that you really cannot go at it alone really add to the experience.
Graphics: Engine is starting to show it's age but it performs fantastically, and provides a great atmosphere for the environments.
Gameplay: Already went over this, but anyway, utterly enjoyable. The "AI Director" is devilishly awesome. He knows right when you're having issues and then exploits you until your whole team is on their backs getting their nipples ripped off by hunters.
Audio: Well... this is the first game I can remember in which you are actually at a huge disadvantage gameplay-wise if you aren't paying attention to the audio. There are cues for just about every event. Dozens of them. Fantastic.
Longevity: Great. Only four campaigns, but they are all greatly replayable. There is DLC on the way, but Valve has a tendancy to take their time. Survivor mode sounds awesome and Versus is great with an orchestrated team.
a must buy for any fans of coop, zombie media, or just great action games in general.