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.

I want to particularly commend what may be the very first taste of psionic crunch in a Paizo product. I was hoping for more, specifically with regards to Aboleths, but I understand the caution. For a hard core psionics fan, it's more of a small sample, and for psionic foes, the crunch is presented as a very small "sample" stat block, which shouldn't be too disturbing.
It provides a strong overview of basic features of the area, but I was hoping for more details about Aboleth cities and about the creators of Orv.

If one already knows what the Underdark/deep/gloom, etc., is, then I would say only those who are interested in the Golarion version of it ought to purchase the content-light product.

Less than the sum of its parts.

There is only one complaint I have with "Into the Darklands" - it's too short.

Yet, despite just 64-pages it's so crammed with awesomeness that it makes WotCs Underdark book look silly. There are so many amazing ideas there... You won't be missing the absence of certain emo drow ranger or the Mind Flayers.

I was really afraid about how Paizo can try and make their version of Underdark "similiar yet different" from the Wizards' one. But once again, they pulled it off, thanks to great talents of James and Greg.

Having read the PDF I can't wait when I get to throw my players into the deep below Golarion. There's just too much cool stuff there. Thumbs up !