I actually enjoyed this book much more than I thought I would. That said though, I did see some serious flaws in it, most of my views on that may just illustrate why fantasy novels in general are not something I generally read.
So I will start with what I found the most annoying. Even in the most fantastic
world created in a fantasy novel I expect it to have its own logic, a consistent set of rules. In my opinion this novel failed in the important matter of under what circumstances and to what extent the population of “London Below” can be seen and noticed by the population of the real present day London. When Jessica and Richard first encounter Door on the sidewalk they both immediately see and recognize her, completely at odds with the rules for to what extent real Londoners notice London Below people elsewhere in the novel. Obviously the novel goes no where but for this, but still here, and through out the novel, this crucial aspect changes arbitrarily as needed by Gaiman to further the story.
I really did find myself drawn into the story though. I especially liked the angel Islington aspect of it; the idea of a fallen angel akin to Lucifer was great. In fact I came away wishing that there had been a lot more development of that story; how Islington became evil, how he became imprisoned, the role of the Black Friars, etc. In fact I would have liked a lot more development of how London Below came into existence and why it was the way it was and why its relation to modern London was what it was. For instance, London Below seemed to have been populated by and culturally tied to the historical period of somewhere from Medieval times up to approximately The Renaissance. Why just that segment of history? On the other hand going into such complete development probably would have at least doubled the length of the novel and I would have probably been now complaining about its length and slow going.
I had a problem with the ending as well, unless there is a sequel to this novel, or one is planned. Otherwise it seems tacked onto the book and at odds with the rest of the book. Or is it just standard for fantasy novels that once an ordinary person makes a trip into a fantasy world he or she will never be the same and will want to return to the fantasy world no matter how dangerous and unpleasant that journey had been?
Finally this is sort of an off-the-walk comment, but it was one of the first things I made sure to make a note about as I read this. That is the banter between the murderous Mr. Croup and Mr. Vandemar. I just immediately thought of similar banter between the pair of assassins, Mr. Kidd and Mr. Wint, in the film Diamonds Are Forever (Sean Connery's last outing as James Bond):
Mr. Wint: The scorpion.
Mr. Kidd: One of nature's finest killers, Mr. Wint.
Mr. Wint: One is never too old to learn from a master, Mr. Kidd.
So I enjoyed reading this, but it did not make me want to read more Gaiman, nor more fantasy novels.