This book is fantastic. Even though it ostensibly contains options for expanding the Advanced Fighting Fantasy game, I would consider it an essential addition to the core rulebook. It manages to make various aspects of combat in the AFF game much richer and more engaging, while still retaining the simplicity that is fundamental to the system.
In addition to providing more choices to players in desgining and playing their characters, this book contains a lot of information useful to Directors in planning and running games, such as will help to keep even experienced players a little more on their toes.
I was also pleased to see material applicable to characters other than 'pure fighters' - for example, the inclusion of rules for combat magic, and mounts of various kinds.
Finally, the appendix at the end indexing rules and options from all the rulebooks published so far is extremely useful.
In summary, if you're a player, get this book even if you're not playing a straight fighter. It will enhance many aspects of your game, whatever your character. If you're a Director, just get this book - you absolutely will not regret it.