For the content of the book, I say 5 stars. But I really struggled to follow the authors thoughts at many points. Because of this, I won't be recommending the book to beginners to the path of spiritual formation.
I've read quite a bit on the topic and that helped me to not get bogged down in specific sections. Because I had a grasp of the concepts coming in, I could reconstruct the logic. I do think the logic is there and Dallas Willard certainly tried to explain it. Frequently, the summary was clear but the details scrambled my brain.