This book contains good thoughts & principles for running a game as a "no-prep" GM. However, my criticisms are: one, there is still a decent amount of prep being introduced in the book (albeit with thought taken to make the prep minimal). Second, without any sort of random tables or actual content for your game, you have to turn to other sources to truly be a "no-prep gm".
Perhaps this book might be good for somebody who never thought of being a "no-prep gm" before and has no experience doing such a thing. The reality is, most GMs I know have run at least 1 campaign with no prep whatsoever, and most of the games go just fine. So, I really feel this book had good intentions, but fails in the application.
[2 of 5 Stars!]