This book started out with a strike against it as the Holmes vs The Ripper idea has pretty much reached a complete saturation point. In fact, often while reading this novel, I felt that I had read it previously only to find a new bit here or there that would have left enough impression on me to precisely remember having read this book or not.
That being said, the quoted reviews on the back cover claim the book to be "controversial", a claim with which I can readily agree. As a traditionalist, I found the ideas proposed in the book a bit unsettling. But, I do on some level appreciate a novel alternate ending to the Holmes story.
I've read other reviews on this book in GR that completely cringed at the ideas about Holmes that were proposed in this novel and I've read others that praised the ideas. My suggestion would be this: If you are open to some radically different ideas about Holmes, read this book. If no, then skip it.