The product gives a large number of new creatures to add to a campaign and presents many interesting features and powers for them. As another review notes the "Seeds" for using monsters are also useful. The "quick template" guides for making human NPC's into dwarfs etc are also handy.
It has two flaws: significant internal duplication of content and "a step backward" on guidance previous products have about monster actions.
Regarding duplication, monsters are presented one in a "mostly narrative" section and once in a "mostly stats" section. So you have to flip back and forth to see both. Doing this you will see duplication of the text on special non-action card features and abilities such as Night Vision, Swarm features etc. (e.g., you will read twice what each of those means). Leaving aside the "divided content" approach, adding the duplication up across every creature, this amounts to considerable space being dedicated to repeating content rather than giving something new/additional.
Monster actions, up until now monsters have been published in formats where you see a group of 4 or so, which may share a very basic action and then each have specific ones. The Guide adds more possibilities so you can vary creatures (not every Gor Beastman is the same etc.). However what it doesn't do is give any advice on staring points and how many core actions each creature has. Monster actions are listed completely sepearately. There are lots and lots (great!) and they have traits indicating which monsters they go with - so you must review them and determine which ones to give a monster from scratch. There is no "basic starting point allocation of what actions a Vampire gets" that lets you quickly "run with it". This could have been provided as a quick index of actions, using the space instead allocated to duplicating the existing content.
So in all, worth the $15.00 but you will have to work harder to make use of the content than I think you should have to work.