This is a great supplament for an underdark campaign. It has great potential for world building and encounter building. It doesn't contain a plot point campaign or specific adventures, but if you are doing an OSR game with heavy cave exploration, it is pretty useful. The useful thing about it is being able to come up with a playable encounter in moments with the handy tables. New monsters, weird civilizations, and differant types of darkness are well worth the purchase price.
The art by Scrap Princess is a love or hate thing. It has a very distinct style and it has the impression the creatures there in are what the adventurers actully see jumping out of the darkenss at them. That give is a indistict look, that I like, but some of my players felt was underwealming. Art is a personal thing, so no accounting for taste.
[5 of 5 Stars!]