Occult Mysteries has been great as a PFS GM understanding a bit more of the ongoing mysteries / controversies in Golarian's history. Does it answer the mysteries? No. And as a GM I'm glad. The mysteries of the setting are meant to be that. What it does is provide GMs with the tools and details to build upon these mysteries and intrigue your players into investigating them.
As far as I'm concerned, this book has exactly one usable piece of player crunch: the lucky number spell. On the other hand, it's a great spell that can be gotten in a wand and be used by most casters in PFS.
As a PFS GM (and a frequent player of Dark Archives characters), this book has provided material for me make several recent scenarios come alive for my players. It talks about the death of Aroden, the Harbinger of Fate cult and the Book of 1000 Whispers. It discusses the mysterious disappearance of the Number 5 issue of the Pathfinder Chronicles. It even has a chapter that compiles the theories of why gnomes left the first world -- a place perfect for creatures of caprice and chaos -- to come to the material plane with its order and mortality.
If you're looking for crunch, look elsewhere. If you're looking for real answers, look elsewhere. But... If you're seeking more information on the great questions of the Golarian setting, this is the book for you.