This particular system is like stepping into a bath full of really cold water -- you're either going to like the refreshing nature of it, or you're going to leap out of the tub with a scream! Based on the Gumshoe system, the game revolves around atmosphere, atmosphere, atmosphere. The rules are merely a vehicle for creating opportunities for the players to act out some truly horrific scenes and events -- very much like a combination of the "worst" teen slasher movies and some of the more esoteric horror written by folks like Steven King and Ramsey Campbell.
The system has been pretty well covered by several of the other reviewers, so I'll confine myself to stating that it's simple to learn, easy to teach and devilishly complex to use effectively -- requiring a particularly skilled group of role-players to reach it's full effectiveness. If you've already read Gumshoe, or Trail of Cthulhu or The Esoterrorists, then you already know more about the system than you'll learn from this book, and large parts of the text are simply extracted from the other publications without any changes at all.
The book is full of guidance on how to run the game -- needed given the way the author recommends the GM interleave different scene types and pull the players into various bits of acting as the game unfolds (one minute they're the lead character, the next an anonymous supporting character, and the next in a regular group scene; add in flashbacks, motivators, mentors, yada, yada, yada, and it's easy to see why this one might be a bit much for the average group of D&D players to simply leap into). The book also contains a fairly horrific example scenario that involves some decidedly "adult" themes. But then, I suppose if you're planning on playing in a truly horrifying role-playing game, you're going to have to get into kinky sex, huge amounts of unredeemed gore, sickening slides into madness and paranoia, and all that goes with them. The creatures described in the game (both of them) are suitably horrific, and should be sufficient (believe it or not) to keep the players fully involved for literally months of play. Since the supplement mentioned in the rules and The Esoterrorists are already out, if you're willing to pop for them, I'm sure you can get plenty more creatures from the Outer Dark to use in future scenarios.
As usual, DriveThru's reproduction of the material is excellent -- highly legible and the images (alas, some are fairly disturbing) are fully viewable. Overall, I give this one a "5," however, based primarily on what I see as a lack of universal appeal, you may want to consider this purchase before making it. While the quality of the work is excellent, it somehow just seems like the kind of play group that would really "get into" this one, might not be the kind of people you want hanging out around your house during the dark of night! :-)
[5 of 5 Stars!]