I've never really read much Neil Gaiman before — I'm not a particular fan of the Sandman series, for which he is most known. But I know some people who are big fans, so I respect his work at least. Neverwhere is only the second Gaiman book I've read (well, third maybe, I think I read that thing he co-wrote about the apocalypse), and I'm pretty much satisfied so far.
Neil Gaiman does fantasy. Not medieval fantasy or sci-fi or anything like that, but fantasy in modern-day settings. Neverwhere takes place in London, a place with which the author seems intimately familiar. Except that his fictional London is divided into two cities -- London Above, which is the normal everyday London, and London Below, a temming network of underground locations where people fall through cracks and disappear. Something like that.
Our protagonist is an ordinary guy, who meets a resident of London Below, and is thereupon plunged into it's mad, mystical world, where the familiar laws of reality seldom seem to apply. The plot is nothing extraordinary, one you may have read in some form or other: Our intrepid hero meets an interesting cast of unlikely allies and they band together on a quest, ultimately saving the universe. Something like that. It's Gaiman's writing that stands out though; his flowery prose and florid descriptions brings the fantasy world of London Below to life, presenting a striking contrast to the protagonist's boring life in London Above.
All in all, a good read. Nothing extraordinarily deep, but enough to make me interested in reading more Gaiman.