The setting/premise is a very flavorful and engaging one: By staying awake past the point of madness and exhaustion, your eyes truly open and you see the city "beneath" the city. There's a section on "sources of inspiration" in the back that name-drops Neverwhere, Dark City, etc., and I'd agree that this RPG does an admirable job of capturing the feel of a dark, surreal world layered over our existing one. The rules are a simple dice-pool affair, but instead of being based on skills and attributes, the number of dice you roll depends on how much Madness and/or Exhaustion you're willing to draw upon for the action. Both can give you amazing abilities, but at the risk of going just a little bit nuts or wearing yourself too thin. The game system is obviously built from the ground up to support the central themes, and it lends a synergy that ties the whole experience together. My only complaint is that the writing can be a bit quirky at times -- for example, the book opens with an Example of Play that was a bit hard to follow -- but only to the point where you might have to go back to reread a section after you've read the REST of the book for context. Strongly recommended.
[5 of 5 Stars!]