As it's been pointed out, Neverwhere is a novel taken from a TV series. And the plot has been changed several times while filming, due to actors unavailability, lack of scenery and so on. And you can tell it.
As a novel we notice first that it's losely based on the standard "monomyth" story line, from the beginning through the main character's odyssey to a very predictable and never unexpected epilogue.
The main character is very good, though: a real Everyman, a mr. nobody who actually does nothing until his ordeal with the Black Friars and does very little after that.
The fictional world is also well portrayed, although, having read the british version of the book, I didn't have the explanations put in the american version so I may have missed something. But it's consistent and fascinating. And I think the fact Jess and Richard actually saw Door is consistent too: as she later explained, she opened a specific and special "door" while escaping the two killers. Having Richard come in contact with the girl, it's logic that he can see and interact easily with the two killers.
The novel is a real page turner, hard to drop dow until the descent in Down Road, despite its minor plot flaws, like the Varney episode: the killers corrupt a to-be bodyguard and after he fails, they kill him for having lost the bid to their actual accomplice... And given how easily they deal with him, what's the purpose to corrupt him in the first place?
In the last chapters, the novel deteriorates. The whole final confrontation simply does not stand: Hunter could have led the party to the Angel's nest protecting Richard from the vampire in the meantime instead of leaving him behind at her mercy (she knew the lady was after Richard's life), while Croup and Vandemar could have followed the company from a distance intercepting the returning marquis. The "good guys" would have come to the Angel, and the girl would have willingly and happily opened the door for him. End of the story.
But no. A ten thousand years plan fails because its ideator and his thousand-years-experienced hitmen reveal too much too soon, and at the crucial moment they fall for the simplest psychological trick made up by mr. Nobody, who, along the lines, has bested thousand of experienced warriors, knights and hunters in confronting the Beast... All having put the highest bet on a "open the door or I hurt your friend" without anticipating any trick on the girl's part...
Sorry, Neil. I just don't buy it.
Bottom line, I give it 3 out of 5 stars: it's a good entertaining monomyth novel, with a late coming of age theme, some good characters, a couple of lovable bad guys, and a very good fictional (under)world.