I bought this product primarily for Chapter 3 of the guide, "Creating the Underworld." In this chapter is two parts: part one on the physical and flora/fauna characteristics of a particular region; and part two on the characteristics of civiliations in the underdark. From my Traveller days (1980's) I enjoyed the planet creation rules that allowed for making varied Sci-Fi worlds. The Underdark Adventure Guide provides 9 physical and 10 cultural characteristics that can be applied to the underdark regions. By adjusting these characteristics, one can create a myriad of underdark regions.
On Chapter 3 alone, I would give this product 5 out of 5. However, it also comes with three other chapters:
Chapter 1 - Underdark Characters - details 8 races for player characters: Deep Dwarf, Derro, Drow, Duergar, Gloom Barbarians (humans), Half-Drow, Svirfneblin, and Troglodytes. Prestige classes, feats, and new equipment is also provided.
Chapter 2 - Underdark Creatures - presents multiple new monsters.
Chapter 4 - Locales of Interest - presents 11 locations that can be placed directly into an underdark setting. Also provided is a 100 mile by 150 mile map of the underdark with all 11 locations sprinkled across it for use in an underdark setting.
5 out of 5. Good stuff all around.