This book is a steampunk alternate history western, which sounds suspect, but actually it worked pretty well. The book had a LOT going for it in terms of creativity. The premise was truly creative, and the interesting ideas kept going all the way through. The pacing was EXCELLENT - the world and the characters were slowly revealed throughout the book, so that the feeling of discovery continued through to the end - just like it should in good speculative fiction!
That said, this book wasn't really to my personal taste. The premise involved too much NASTINESS - the book is all about human failings, and the destructiveness of hate and fear, and about how even our best urges turn wrong....that's not what I'm looking for in a fiction book! I also wasn't impressed by the storytelling - either there's a sequel coming, or the story ends at a very odd point. Since I like to read for character and story, this book didn't rank high for me. But in terms of world-building and originality, it's sublime.
Note: I have since heard that there is in fact a sequel coming. Thanks to Erica, and to the library's wiki!