First off, I can't believe someone hasn't made this into a movie already. It would be kinda great.
Second, just a fun, great read. I think Gaiman often doesn't get as much respect as he deserves for his writing, because of the types of novels he writes, but have to say the man can turn a phrase.
Recommended. A bit overly British and probably good if you know London a little bit since it does deal with the London tube and geography a bit (and thus the 'vested knowledge' of certain stops and neighbourhoods), but good nonetheless. And despite how scary London Below sounds, I think I still prefer it to London Above.