I would honestly like to give this book a 3 and a half star review. It had great ideas for natural hazards. I have already used several in my campaign. (The flash flood was priceless.)It also gives a nice overview of some monsters one might encounter. Also has various molds, fungi, and other hazards.
I use my own world setting, but the info on the Golarian underdark was informative.
My only issue is that since Paizo has released their own campaign with new rules, monsters, and XP; this book is outdated in that regard. However, despite that it is a great book that gets into the underdark very well.
PS. Looking for a good book of monsters and variants for the underdark? I highly recommend Dungeon Denizens revisited.