This is one of those books. I'd rate it alongside other great RPG books like the Guide to Glorantha and the Temple of Elemental Evil. It makes you itch to start playing a game.
Well written and full of superb ideas that you can use in any underground campaign. There's a good dose of revulsion and body horror.
It includes useful mechanics for climbing that feel like the person who wrote the rules has climbed up a rock face. Then distilled that knowledge down into a quick flavourful sub system.
The verticality and variety of the map generation system will keep you and your players well entertained. Some of the locations are truly strange.
The monsters are similarly varied and some of them will leave your players thinking "What was that?"
If you're thinking of running an underground campaign, buy this book.
[5 of 5 Stars!]