Looking through this, I have to say it seems to be mostly a repeat of what I have seen from WotC's Dungeon Master Guides. I clearly received the wrong impression for this book.
The saving grace from getting just 1 star from me were the tables and the NPC write ups. Other than that, I didn't get what I really was expecting from this book. Which was in depth gaming mechanics for some tools GMs really need.
For example, on the sections for Space Travel, Future Tech, or Time Travel, there's no solid game mechanics for running these in your games. Just advice. These are the sorts of things, even if only for one adventure, a GM needs to have on hand. But they weren't available.
The Gambling section was another. I wholeheartedly got this in hopes there'd be a few pages worth of solid game mechanics for running Gambling scenarios and using the Profession (gambling) and other skills for the mechanics.
To use as a comparison, Dungeon Master's Guide 2 from Wizards of the Coast provided some decent game mechanics for "Running a Business". I wanted to see more game mechanics for scenarios a DM wants to run than a half a book of advice. I was sorely disappointed in this.
Maybe something for Paizo to consider if they decide to do a "GameMastery Guide 2"?