Absolute classic campaign! 5/5 stars.

One 4 Nine is a gem. Its levels are set in imaginative, varied, and detailed environments. The level design itself is perfectly up to official Left 4 Dead standards, giving you plenty of nooks and crannies to explore (and rewarding you for doing so) without ever feeling like a directionless maze.

The campaign also has a balanced difficulty, a reasonable length, and a sense of humor. It even has ways of surprising you, like an optional puzzle in Map 5 and the "bruiser brothers" in Map 4.

The only complaint I could possibly muster is that the finale is a small difficulty spike due to a lack of health items. I guess it also stinks that the Survivors never talk to each other, but still, this campaign is a must-play.