I've glanced at it so far..
I"m really liking the expansion on little game situations (I really like the idea presented for making chase scenes!).
As a DM making (and remaking) stock NPCs for a campaign setting, the NPC Gallery is an AWESOME starting point for a huge number of character types. It's so much easier to grab a ready-made stat block and tweak to the flavour I want, instead of building from scratch all the time.
As for my actual review (keep in mind I've only skimmed through this thing)...
I'd say about one third of the book could be seen as GM-trainer stuff. New GMs will eat it up, while old GMs will probably skip over the majority as they already know most of the ideas and warnings presented. There were a few things that made me think, despite my years of DMing experience.
Another one third or more of the book is great for homebrew game building. From world building to adventures. While there's a lot in there that I already knew and considered as a DM, I'm finding stuff giving me ideas. It reminded me a lot of the Cityscapes book (although that had more on the specifics of different cities, etc).
I'm sure I'd use the charts for creating a last minute villain or other NPC if my players surprised me in a homebrew game. The chart for "random adventure party names" made me chuckle.
The last portion would be the specific crunch; rules to use for things like gambling, or addictions, and NPCs.
While I can see not everyone finding everything useful in this book (that's fairly rare in any case when dealing with add-on books), I can see most GMs finding something they really like in this book.