I'm not going to review the rules part; you're all familiar with d20, love it or hate it, and that's not why I bought it. No, the reason I purchased OGL Horror was to mine it for its horror rpg advice from Gar Hanrahan (who, to my knowledge, so far has written four great horror adventures; Black Bag Jobs, Invasive Procedures, Arkham Detective Tales and Zalozhnyi Quartet).
And for 5 bucks, that chapter on GM'ing alone is worth buying this book for. "Turning the screw" (pages 181- 203) is one of the best sections on methodical horror game mastering and creative horror story telling I've read, actually.
In addition, the two chapters on the use of madness in an horror game and the chapter on supernatural ability and spells (i.e. pages 148-169) are also interesting if you, like me, are only mining the rest of this book for things to scavenge into your own favourite system (...which ought to be Chill 3e, by the way).
Anyway, with my limited interest in OGL Horror, what did I think could have been done to better effect? Well, the sample campaigns at the end felt somewhat flat. And I missed some interspersed examples from actual play to bring this book and its advice to life, really. Also, in the monster chapter, it felt like psychological aspects in the design of and forensic taxonomy to identify the paranormal entities were lacking. But as a d20 game, it's probably fair to assume that OGL Horror aimed for a somewhat younger audience than, say, the excellent "Horror Recognition Guide".