I bought it a few weeks back. It is a collection of optional rules mostly designed for a primitive or wilderness setting. The pieces are independent, so you can use as much (or as little) as you like.
I liked some of the skills section; some good mechanics on how knowledge (monsters) can work, and quite an expanded set of uses for profession (herbalist).
One of the strengths of the book is its treatment of poisons and diseases. Poisonous plants are given d20 definitions, and there is a lot of detail- what plants a poisonous plant might be misidentified as, for instance.
The Primal Codex defines a whole lot of conditions in d20 terms; nausea, sore throat, drowsiness, headaches (of various intensities) and so on. It allows a more detailed description of the symptoms of poisoning or disease.
I liked the section on wilderness traps. The Craft (Trap-making) DC's are there, and the DC's to spot and disarm them. The damage they do, the time it takes to make them- and well-illustrated, too. Lots of ideas that you as a DM can inflict on your players. If you liked Traps and Treachery, you'll like this section.
Some bits, like rules for fighting in water are kind of nice. Others (descriptions of different execution methods) are kind of disturbing.
Weapons, armor and equipment for a pre-historic setting, new classes (huntsman, brawler, shaman, witch doctor) and lots of new spells and monsters. Magic items too, of course.
Many of the spells are logical extensions of existing spells (variations of Ray of Enfeeblement, for instance, which affects different ability scores) or which impose a condition on someone (a spell to make someone exhausted, for instance.)
The monsters are great if you want to create an African or South American feel to your campaign. Or if you need stats for a fox or antelope or hippopotamus or something. It would be nice if there were pictures of the fantastic monsters, though.
It's a big book, and there is a lot of material. I think I touched on the main points of interest.