The only real positive thing I can say about Neverwhere is that Gaiman has built a genuinely interesting world. The settings are dark, gritty, and imaginative. The concept of having a parallel world composed of those who've 'fallen through the cracks' is an attractive idea and Gaiman does a great job of showing the interactions (or lack thereof) between our world and Neverwhere. The cultures of the various underground groups are fun to read and are portrayed with enough detail to add depth and believability without detracting from the story or slowing it down. 
 Other reviewers, here on Goodreads, have commented on Gaiman's 'beautiful writing' but, I must admit, I saw little in the way of beauty. In fact, there is some decidedly unattractive writing in this book and it isn't just sporadic lapses but there is a constant stream of it that emerges on every other page. Gaiman's excessive reliance on adverbs really doesn't help. Adverbs are fast, cheap ways of explaining something (which is fine) but when used excessively they interrupt the flow of the story with unnecessary detail - especially when used to qualify dialogue which Gaiman does to a painfully excessive degree. This means that he hardly ever lets his character's speak for themselves. 
 One of Gaiman's favourite qualifiers is the word 'helpfully'. Whenever someone (usually Mr Croup or Vandemar) says something that is obviously sarcastic, Gaiman can't resist mentioning that it was said 'helpfully'. When someone says something that is obviously said in anger, Gaiman says that it was said 'angrily'. When someone says something that is accurate, Gaiman actually says that it was said 'accurately'. The author even uses the words 'extremely grudgingly' (more than once) to describe speech. Not only does that sound quite clumsy but it is completely unnecessary in terms of clarifying the dialogue. But Gaiman's over explaining isn't limited to extraneous adverbs. For example on page 80 there is a stand alone sentence that reads: "...something whispered and shivered and changed...". Saying that something changed doesn't really help much. It's just more empty and unnecessary words that pepper a lot of the pages of this story. Again on page 34: "Mr Croup sighed, in what he obviously imagined was a rather wistful manner." If it was really that obvious then why did you have to explain it? 
 And with descriptions like 'blood dripping wetly' (as opposed to dripping dryly?), can it really be said that Gaiman's writing is beautiful? I can let ugly or clumsy words/ phrases slide by if they only happen every now and again but with Gaiman, I found that these kinds of things are prolific. Of course, it may just be personal taste, but his similes are generally quite weak as well e.g. "Mr Croup looked at Door, and he smiled like a fox on really nasty drugs". That's such a vague simile as to be borderline useless. 
 The humour (at least in the author's preferred text) sounds like someone trying far too hard to be funny e.g. "It had been like watching Emma Peel, Bruce Lee and a particularly vicious tornado all rolled into one and sprinkled with a generous helping of footage he has once seen on a wildlife programme of a mongoose killing a king cobra." A lot of the time, his excessive and clumsy descriptions interrupted the humour so that it read as someone who is constantly having to explain his jokes to the reader to make sure they got it. If he'd dropped the descriptions and let the jokes stand by themselves then it might have been easier to digest. 
 On top of largely unattractive writing is the passive protagonist, Richard Mayhew. For ninety percent of the story, Richard is barely more than a fly on the wall, tagging along with a group of lower Londoners as they struggle after some vaguely defined quest. Richard cries and moans for most of the journey and barely participates in any of the decision making. Near the end of the story, Richard performs a couple of token gestures as if Gaiman realised at the last minute how weak a protagonist he'd created. Only one of these gestures actually has Richard actively doing something. And when he does actively participate in the story it only serves to diminish the qualities of one of the other characters and throw all their credibility into question. I'll avoid spoilers here, but near the end of the story, Hunter, who we are lead to believe is the best hunter in the underworld is outshone by the meek Richard in a way that only seems counter to everything established about both of these characters. Perhaps if Gaiman had written the scene with a bit more substance or drawn it out a little longer to really make both Hunter and Richard's actions believable and in line with their characters then I could have gotten on board with it but this is not the case. 
 Overall, the story is shallow and predictable with little to no surprises. We don't learn much about the mechanisms that are driving the story forward beyond the fact that Door's family was killed. There are some vague references that her father was trying to 'unite' the underworld and a few moments of insight into Door's relationship with her father but these moments are fleeting and their effect is minimal. 
 Neverwhere is at least an easy read. While the cast and story are very forgettable, the setting might have a bit more staying power.