I don't think it's been underrated, in critics' circles, for 15-20 years or so by now. Since the tales about its production, and the related art, have fully emerged, it has been widely re-evaluated - also because Lynch has gone on to become a bit of a sacred cow after Twin Peaks and his later work.
The original series set the bar for thriller tv series from then on - both on the good elements (always one more secret, one more cliffhanger; living on the edge of fantasy) and the bad (making up stuff as they go along; disappointing finale; unceremoniously cut).
I have only watched a couple of episodes of series 3 but by now I've seen enough Lynch to dread his output, narratively speaking.
