Com excessão de algumas meta tramas do cenário, mestres narrando Second Darkness ou fãs do Underdark (os fãs de drows conseguem melhores informações comprando as aventuras de SD) as darklands são um cenário bem aparte do Inner Sea e portanto bastante opcional. No entanto este livro contem praticamente tudo necessário para fazer campanhas em ambientações subterrâneas e isso o torna uma fonte de referencia necessária (já que a maioria da informações com excessão das fichas das criaturas, não se repetiu em mais nenhum livro). Também é importante para saber um pouco mais sobre raças e criaturas que são grandes antagonistas e outros cenários e que em Golarion até agora apareceram pouco.

Once again, Golarion breaks from the pack. Detailing the lands below, a complex somewhat orderly ecology exists miles below the surface. Serpent Men and the Culture of Ghouls replace the Mindflayers as rivals for Drow dominance. A must have suppliment. Check my full review: Into the Darklands

Then you will love this book. It takes the very best hollow earth 'bits' from the past hundred years or so and brings them all together in the vast, sprawling, and maddening Darklands of Golarion. It has ghoul kingdoms, demon-worshipping drow, hollow earth vampires, Howardian/Clark Ashton Smithian serpent people, massive caverns with dinosaurs, and even gugs. You can't go wrong with this!

The first thought on Paizo's underdark treatment is they've done a very good job of keeping things segmented/customizable. The division of three layers, and regions reminded me a little of Ravenloft domains in a good way. The GM is not obligated to have everything in the mix but rather to focus on what they want.
Now to the content itself. There's some neat stuff, love the treatment of the derro, the Lovecraftian/Hodgson touches of the the third layer with gugs, ghoul empires and prehistoric beasties. Oh, and neothilids!
A great hollow earth pulp feel.
My main issue is with the Sekamina, the 2nd layer. Paizo's done a commendable effort with the drow but I think they would have been better to jettison entirely. Yep they're an iconic villain but thanks to Salvatore and FR I wouldn't touch them with a ten foot pole.
Still all and all a good starting document to springboard ideas for campaigns/adventures. Recommended. PDF is good quality.

As another reviewer said, there simply wasn't enough but you get enough for the asking price.

The separation of three different Underdarks was well done, as was the reimagining of the derro, deep gnomes, drow and druegar.

Lots of great ideas that could easily flesh out the more classic D&D Underdark and as such is useful even for those not running Pathfinder.

There is a picture of an aboleth that is simply "gorgeous" in that it is genuinely creeping as all hell.