Dark Wood, along with Buried Deep and Wild Ride form the "triple crown" of custom campaigns for L4D2. My brother and I have enjoy playing buried deep for its long, but enjoyable maps. Difficulty can range from easy to hard depending on player skill level, but personally I would rate it as balanced if playing with a friend and hard if solo (since the bots suck tbh).
TL;DR: Stop reading this review and download it already.
Long, well-balanced and truly a great adventure.
This review was posted before the latest release.