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