Catherine Fisher on top form, technically, gives us a story about a bargain for a teenaged girl's soul. Inspired by Dr. Faustus and the alchemical quest to create gold from base metal, this story's primary strength is its characters. They are flawed, sympathetic, real. Indeed I think this is Fisher's great strength as a writer; her protagonists are completely convincing human beings. (Fisher has used all sorts of mythic and religious inspirations; classical Greek, ancient Egyptian, Celtic, Arthurian and Norse for example and has generally come up with an interesting twist or interpretation - this is no exception.)
The ending became predictable too early but you certainly wouldn't have guessed it at the halfway point where everything takes a surprising turn for the more interesting.
I met Catherine Fisher once and she told me that she chose the name Lord Azrael for a character in this book, only to read Northern Lights and find Pullman was slightly ahead of her in using the name - it was too late to change it, though, and it does suit the character.