Drow of the Underdark had lots of potential. Sadly, it fails to live up to that potential. The book follows the now-standard book format, with an intro, options for Drow characters, new prestige classes, new magic, monsters, etc. Nothing too out of the ordinary here.
The feats include a fair amount of drow-specific feats. Some of these are pretty nice - Knight of Lolth gives a drow advantages while fighting from the back of a giant spider, for instance. The weapon style feats include the typical range of schools (one for each noble house in the Vault). A few more vile feats, and other goodies round the list out. Skills also are expanded to address some drow-specific traits (like the drow sign language).
Prestige classes include some interesting options, and the monsters include creatures both familiar and new. New magical goodies give the drow a lot of fascinating options.
However, the book fails to really live up to expectations. With 200+ pages devoted to a single race, the book contains a lot of filler. There are lots of pages giving GMs ideas of how to handle or run drow. While there are some interesting tidbits here, but a gamer has to do some delving.
The prestige classes include several that are drow-specific that shouldn't be. If the Arachnomancer isn't drow-specific, why should a Demonbinder or Insidious Corrupter be?
The monster section is a disappointment. Aside from using the new monster format, complete with sample encounters, many of the monsters are either reprints, or worse yet, simply adding class levels to drow or other humanoids. I would much rather see more fascinating monsters than a "Goblin Flesh-Herder" (goblin Cleric 3).
Sadly missing is any discussion of the other deities of the drow (outside of Lolth). Eilistraee, Vhaerun, and Kiaransalee should have been included somewhere, even if they were originally deities in the Forgotten Realms. The prospects of secret clerical politics and cults could have added a lot.
Also missing is much about some of the underground flora and fauna. How about alternative familiars for drow mages (pet Spider: +3 climb)? Properties of some of the weird fungi? Drow are attracted to magical enamations underground - it would have been nice to explore some of the other effects of such places.
If you want to run a drow campaign, this can be useful. The 2nd edition Drow of the Underdark contains a lot of useful material too - probably more meaty material in a small book. But this book devoted 200+ pages to a single race - compare that to the coverage the remaining elven races received in Races of the Wild. This book should have been condensed as a large chapter in a Races of the Underdark book.