I think this book should not be rated on what people think of Dr. Spencer and/or his views on Islam. It should be based purely on the essential argument highlighted in this book and key aspects of Islam history.
I have read most the Qur'nic verses and Hadiths in Arabic [originally written] referenced in this book, therefore I view the book to an honest, and accurate record of Muhammad's life and his career as prophet. All the materials are based sources widely [if not fully] endorsed and recognised in the Islamic word as the source of truth about Muhammad.
I genuinely recommend the book, it is necessary that people understand the thinking and motives of Jihadist around the world, and most importantly what are the core issues preventing Islam to be a spiritual-religion rather than [as it is now] a political ideology requires to remain in conflict to exists.