I am possibly easy to please on this one, as I really Jon Ronson's work. He and the likes of Adam Curtis (mentioned in this book) are two of the people who keep documentary journalism relevant and interesting today. 
 I read this in two sessions, probably five or six hours total, and was thoroughly engrossed pretty much all the way through. 
 The only reason I've given it four rather than five stars is because I wasn't particularly grabbed by the David Shayler chapter. This might be partly because I am already familiar with his story, as are a lot of people in the UK (ironically, partly due to his past appearances in Jon Ronson's stuff), but mostly because it didn't feel to me that it "sat right" with the rest of the book. 
 Shayler is quite clearly - to use an Alan Partridge term - a "mentalist", but does he really fit with the description of "a psychopath" given the terms of reference that run through the book? He's certainly got a lack of empathy (the 7/7 bombings nonsense he engaged in), but how much more? Or maybe that's the point Ronson was trying to make in introducing Shayler - that not every, err, "mentalist" is a psychopath? 
 Anyway, a very good read, and one that I struggled to put down once I'd started. 
 Slightly irrelevant side note, but you know those authors whose books you can't help but read in their voice? Stephen Fry, for example? I imagine Ben Elton would be another if you read his novels. Jon Ronson is another one of them. 
 I hope the Hare psychopath test never gets expanded to include "reading books in the voice of the author".