Interesting. That's the one book I would strongly recommend against. Amongst other things, it has the worst execution of a group of prestige classes in any product ever. Every single BAB and saving throw progression for each class is unique and, as far as I can tell, random. Overall I rate it one of the worst purchases I have ever made.
OK, that might be just a crunchy thing and fluff is more for you: I would instead recommend a 2E product for the Forgotten Realms, Drizzt Do'urden's Guide to the Underdark as being a better (and cheaper) source of ideas of how to populate the Underdark with interesting sites and encounters (and has nothing to do with the angsty drow of the title). And it doesn't have radiation and a class that uses radiation.
3E's Underdark, also for the Forgotten Realms, is good as well. Both products have a high degree of utility outside of the Forgotten Realms.
My enthusiasm for prestige classes, new feats, etc. is at an END. I simply cannot use one more of either. I hate to purchase a book and have a ton of that. The 3.0 to 3.5 difference doesn't bother me as there's such a minute change, it's still quite gameable.