I read the original Nightworld many years ago, and just today finished the new edition, heavily revised to punch up the role of his parallel-series Repairman Jack character. (There are a couple amusing allusions to how the author is now a little embarrassed by that name.)
As a horror novel, Nightworld works pretty well. Wilson has dreamed up some pretty creatively horrific monsters and a fairly badass fate for the world.
As a piece of writing, Nightworld doesn't fare quite so well. Wilson is no poet. When I read his earliest novels he felt to me like a much better writer. Either I was enjoying the stories so much that it skewed my perception of his talent, or they WERE better-written and he's just gotten lazy or sloppy as he's cranked out sequel after sequel over the last couple decades. (I noticed a similar phenomenon with Robert K Tanenbaums Butch-and-Marlene series...great early, awful now.)
The tiresome ending is ripped right out of Ghostbusters 2. I hated Ghostbusters 2. Nightworld is a serviceable wrap-up to the series but it's no great shakes.