I was apalled by the lack of official fumble rules in the Pathfinder rulebooks, but with this deck, my players can now fail like they've never failed before! There are many possibilities offered by the good amount of cards and the fumbles that result from casting spells (like making your target bigger instead of wounding it or accidentally summoning a hostile creature) are especially fun and devastating, a bit similar to wild magic!
With this book, any player or GM will be able to customize its character to turn it into what he really wants it to become. The new racial skills are great, the six new base classes are awesome and the huge amount of feats and spells just make it all better.
One thing I've learned by game mastering is that you can always learn. Every game session brings its load of questions and problems, and the GameMastery Guide is there to answer them. Game mastering is a hard thing to learn or teach, but this book is full of helpful tips that every GM should know. If there had to be a bible of GMing, I would choose this book for its great, straight to the point advices. Also, the tables are fantastic and enable a much faster play!