You probably don't need me to tell you that Neil Gaiman is great. If you have read anything by him (or have friends who have), you know already. Neverwhere features many elements from the rest of his work-- shady characters, horrific details, dark humor, and lost characters.
The story follows Richard Mayhew as he leaves his secure and boring life through the gaps into "London Below." The setting was my favorite part of the novel. This underworld is filled with forgotten characters, shady guides, and talking rats. The descriptions are clever, clear, and creative.
While this was not my favorite Gaiman work, it's definitely worth a read. Of his other novels, I can't recommend The Ocean at the End of the Lane enough. American Gods is worth the hype and the audiobook has a full cast. It's a great road trip book. The Sandman graphic novels rock, as well.