This book is non-essential for playing the (excellent) new Pathfinder ruleset. It is in essence, years of Games Mastering experience distilled into one volume. If you have been roleplaying for any length of time, most of the stuff in here won't be new to you. At first it seems like teaching Grandma to suck eggs, with advice on whether to have food at the gaming table, and keeping young children out of the room (unless you want a good howling banshee effect). A little further exploration reveals that many of the eggs in here are solid gold.
The 'GamesMastery Guide' is great for dipping in and out of. Any long standing GM is probably unaware they have become jaded. Reading this book, I realised how often I ploughed the same furrow with just enough variation to keep things interesting. There are countless ideas here on world building, campaign structure and alternative adventure themes, that even the most experienced GM can use to improve their games. There are also some handy NPC tables for just about any situation. Very useful for when the PC's throw you a curve ball.
There isn't necessarily much new in here (It is very similar in content to the D&D4 DMs guide) but it is well packaged and interesting to read. If you are strapped for cash and your games are working, then don't bother buying, but if you are feeling flush, and need a little inspiration the 'GamesMastery Guide' is a worthwhile purchase.