Occult adventures introduces amazing new classes like the Kineticist, the mesmerist and the psychic. The flavor is fantastic allowing several concepts that were in need for a long time. There are new mechanics for rituals, mind battles, unlocking chakras, among others.
The best part: the Kineticist (the jewel of the book), psychic and the whole new spellcasting system, the mesmerist.
The regular part: the medium (overly complicated, and always sub-par, poorly emulating abilities from other classes), the occultist for much the same reason.
The worst part: the spiritualist - an underpowered copy of the summoner re-flavored (could be an archetype), the feats and magic items (too few and lacking impact).
Overall: This is a great book, although not one without flaws. New classes like the Kineticist and the psychic are still in dire need of further development (options, feats, archetyypes, talents...). The theme is coherent and well executed, making this one of the best pathfinder books I have ever read. It's a shame it ends so quickly, because it seem to have the potential for several books.