There isn't much left in this world that makes me fangirl like some 14 year old Sherlock groupie. Neil Gaiman is one of those things. He should absolutely be locked up in a museum somewhere as a national treasure.
Neverwhere does everything a good fantasy romp should do and does it with proper spelling. Ordinary sort of bloke gets dropped into a bizarre and extraordinary world and in the end the day is saved. Richard is, as other reviewers have noted, a little bit wet. He is passive at best and spends much of the story standing around trying to convince himself this isn't really happening.
The supporting characters, on the other hand, are delightful. de Carabas, Croup and Vandemar, Old Bailey, Hunter, Anaesthesia, Islington - all are vivid and colourful characters with immense appeal. Door, funnily enough, is not. She does everything right, according to the heroine handbook - takes charge of her own fate, protects herself, thinks on her feet etc - and yet she's still more than a little bit meh.
If one can accept that the two main characters are probably the book's weakest link, the rest is gorgeous. Pure braincandy. The miniseries was a delight and I'm certain that the upcoming radio play (Christopher Lee! Anthony Head! Johnny Vegas! James McAvoy! Benedict Cumberbatch!) will be beyond cool. But. Neil Gaiman's medium is the written word. He crafts text and does it brilliantly. The book is a faithful rather than a slavish novelisation of the miniseries and is well-worth the read.