To me it looks and feels like the Blackreach in Skyrim, but falls flat because it has none of the necessary build-up. The underdark just shouldn't be the 2nd area in the game, and if it needed to be it shouldn't be a place you nonchalantly go to for a quick trek.
But then again, within the first few hours of the game we face Illithids, fiends, drows and giths, so it's not like the underdark is alone in being stripped of its mystique by the 'cool' factor of having it early on.
