Underdark

I both liked and disliked this sourcebook. There's a disconnect between the meaty parts and the flavour parts, so it's value depends on what you're looking for.

The flavour and lore parts are really great. It provides varied details that come together to paint a vivid world beneath the surface of Faerûn. There's a little bit of everything - the aberrations, the high societies of drow and duergar, invaders from another setting, even outer planar visitors. It's a great resource for world-building and underground adventures.

Too bad the meaty parts don't gel nicely. The new races presented are all uninteresting to me - I can't get a “feel” of what they're like (aside from the already well-established drow, duergar, and svirfneblin). They're also mostly high ECL races, which aren't really that nice to play. The prestige classes are boring - they wouldn't be enticing to players, both in terms of mechanics and in terms of role-playing. Some of them have odd nonsensical entry requirements. The items are adequate, with a few new materials and special attributes. The spells felt a bit unimaginative - with a large number of them being greater/lesser/mass variants and a whole bunch of node spells (magic-buffing earth nodes, a feature of the Underdark).

All in all, if you like the Forgotten Realms, and want to get a better picture of the Underdark, then this will be an awesome sourcebook. But if you're looking for more underground adventuring options and how to run more realistic subterranean explorations, then this would be a poor sourcebook.