However, it doesn't have to be, and one of the things that Paizo's book helped to highlight is that the Underdark is a wilderness. Sure, it's an exotic wilderness, with hazards that are quite a bit different than wandering through a forest, desert, or tundra, but it clearly presented the Darklands as a relatively wild place; a wilderness, punctuated by points of civilization here and there.
Cruel, sadistic and unfriendly civilization, but civilization nonetheless.
Anyway, this approach certainly helps to make the Darklands much more palatable to me, and in fact fairly interesting. The other thing that Paizo did was acknowledge the roots of the Underdark concept and return to something more like it. In James Jacobs' Dragon Magazine article "The Shadow over D&D", he acknowledge H. P. Lovecraft's Underworld as one of (if not the singular) influence on the concept of the Underdark. This is primarly (although not exclusively) explored in two of his stories, The DreamQuest of Unknown Kadath, a novel or novella set in the overt sword & sorcery fantasy realm called the Dreamlands, and "The Mound" a longish short story that Lovecraft ghost-wrote for Zealia Bishop.
Besides that, there are obvious nods to other famous underground/hollow earth stories, including Edgar Rice Burroughs' Pellucidar books, Jules Verne's Journey to the Center of the Earth and even H. G. Wells The Time Machine (notably in the inclusion of the morlocks. I've included a picture of a morlock from the recentish movie, just for the heckuvit.)
Paizo were quite upfront about this influence, spelling it out explicitly in sidebars. Some obvious Dreamlands influences include the use of gugs, a ghoul kingdom, giant worms that resemble the bholes (although that's not what they're named here) and a lot of details of the geography in general, like gigantic underground mountain ranges (not called the Peaks of Thok, sadly.) Their Darklands ghouls even hate ghasts, contrary to most D&D implied setting stuff (but consistent with Lovecraft, where the ghasts are totally different creatures that the ghouls hate.) The concept of underground serpent people empires is also borrowed wholesale from "The Mound" (although Paizo serpentfolk don't worship Yig) and there's even a mention or two of a shoggoth.
Paizo's Darklands are divided up into roughly three "layers"; the first being the one closest to the surface, which is very much a wilderness of savages and monsters. The second is the home to various civilizations, including the drow, duergar, the ghouls and others. The third layer isn't so much a layer as it is a collection of gigantic underground "worlds" magically sustained. Some of them resemble Pellucidar overtly (including a false sun, jungles, and dinosaurs all over the place) or other interesting high concepts (the land of black blood, for instance, which oozes a freezing, tarry substance that facilitates various necromantic rituals, or the land of the undead outcast drow house, or the lands of the various worm creatures, or gigantic underground oceans, daemon-worshipping monstrous humanoids, etc.)
One curious observation; the book refers very frequently to Necromancer Games' Tome of Horrors, Revised. I've noticed before that Paizo really like this book and refer to it a lot, but in this book they really outdid themselves to the point that it's almost a required supplement to use this. Not quite, but some of the material would be pretty tricky without it, especially a lot of the random monster tables. I actually don't have that book, although I do have the original Tome of Horrors, done in 3e rules (not 3.5) so I figure I'm covered. Frankly, I'm not a huge fan; it seemed like the book's main raison d'être is to stat out monsters that were deservedly left behind by Wizards of the Coast. It's not quite that bad, as they also included a bunch of new material, but I always found the book to be superfluous at best. Clearly Paizo disagrees, but with their focus on old-school D&D flavor, that's not surprising to me.
Especially for a guy like me, who really quite enjoys seeing Lovecraftiana incorporated into fantasy gaming, but for anyone who has any interest in the concept of the Underdark at all, I can unreservedly recommend this book as a pretty fun one. It's a bit on the slim side, so most of what it details really needs further detail to be useable, and you can get some of that in places like the various Second Darkness adventure path materials (including details on drow society in volume 3, details of drow and serpentfolk cities in volume 4, and details of the land of black blood in volume 6.) But there's enough here to keep me happily exploring, if I'm so inclined, for a long time to come yet.