That begins with the subtitle "A Prophet for our Time." That's certainly not an unbiased opinion.
In short, Armstrong goes beyond throwing out the bath water of some Western total besmirching of Muhammad, to throwing out the baby of a critically-based biography of him at all.
While I disagree with one-star reviewers that this is a whitewash of Muhammad's life, it is selective. Beyond that, when Armstrong does talk about some of the less noble actions of Muhammad, she always tries to put the best spin possible on his actions.
A prime example is of his retreat from his original position of full equality for women in society. The only end view one can take away from Muhammad here is either henpecked husband leading to his first advocating women's equality, or male herd-follower after changing his mind.
Neither view is that of "a prophet for our time."
Armstrong's biography is also strongly biased in its lack of criticality. There's no sense of scholarship in the way of historical-critical wrestling with either the Quran itself or the later hadith about Muhammad.
In short, Armstrong's rehabilitation of Muhammad falls short precisely because it uncritically goes overboard.
I have read or skimmed other of Armstrong's books, and do find a different attitude here, in all the ways mentioned above and more.
It's different enough to leave me wondering if has converted, or is considering it.
As usual, Karen Armstong has taken an important subject and made it highly readable, while maintaining her scholarly approach. This book is an excellent overview of the history of Islam, laying out the prophet's personal history in the context of his times. It provides an insight into the state of Islam today.