In the doctrine of sanctification, or Christian growth, it is customary to distinguish between God's role and man's role. Spirit of the Disciplines is focused exclusively on man's role in sanctification, or his own spiritual growth. As long as you keep that in mind and read from that perspective, this book is dynamite. However, it is useful to point out that this book is not the last word in Christian growth, or sanctification, simply because it does not deal with God's role in our sanctification.
For study on God's role in sanctification you need to look to books like "Transforming Grace," by Jerry Bridges, or "Holiness by Grace," by Brian Chappel. Both books do an excellent job of bringing out the fact that God's grace is the thing that enables a man to engage in spiritual disciplines. Also, Jerry Bridges book "The Discipline of Grace," is probably the best book dealing with both God's role and man's role.
I give these commercials for other books simply to prevent the reader from thinking that Willard's book is the whole story on sanctification. If this is the only, or the main book that you read, frustration is inevitable, simply because Willard does not point you back to the source of our sanctification, or the source of our ability to practice spiritual disciplines, which is the grace of God.
Having said all of that, I heartily recommend the book. I think the most useful thing that Willard said was his comparison of the Christian life to athletics. He has a great illustration of how kids will idolize a major league baseball player (this illustration will work for any sport). They will copy his stance, his swing, his position in the batter's box (I'm thinking of Joe Morgan of the Cincinnati Reds and his famous double elbow pump as the pitch was being thrown), and any number of his mannerisms. However, they will never be able to perform like their idol unless they enter into the same kind of life of their idol. The star didn't get to be a star by performing that way on the spot. He adopted an entire style of life that enables him to perform the way he does on the field. He adopts a certain exercise, diet, and practice regimen that enables him to perform the way he wants to on gameday.
So it is with the Christian - we can not behave "Christianly" on the spot, at a time of crisis, unless we have conformed our entire life to the pattern of Christ. We cannot turn the other cheek, go the extra mile, be merciful, etc., at a time of crisis unless all of our life, before the crisis has been devoted to following Christ. Willard gives Peter as the example of this. When Peter denied Jesus three times, it was not because Peter did not love Christ, for he did. It was not because Jesus had not warned him, for He did. Peter denied Christ because the habitual pattern of his life till that point was one of saving his own skin. Peter needed to develop new habits and a new way of life. I think that in this example Willard neglects the role of the spirit in Peter's transformation, but the point is well taken. We who are accustomed to self-indulgence, must often learn self-denial through a long period of training.
This is where the spiritual disciplines come in - it is through the practice of spiritual disciplines that we become like Christ. Most of the book is a kind of philosophy, or apology for the spiritual disciplines, rather than an explanation of the disciplines themselves. He only spends one short chapter on the disciplines themselves. The bulk of the book is concerned with persuading you and me why we need the disciplines.
In doing so He does make the disciplines seem attractive rather than restrictive. This is one of the great strengths of the book. He shows that disciplines don't restrict freedom, they enable it. Disciplines are not harsh, punitive things, but are the means of knowing and becoming like Christ. In large part, he makes an apology for Christian asceticism. He does a good job of distinguishing biblical asceticism from many of its historical abuses. As someone from the Reformed tradition I have always looked at asceticism as a dirty word, but Willard's take on it is balanced and biblical.
Willard's concern is to show that salvation is unto a new way of life, not merely unto heaven. He shows that the church has pretty much failed in helping people live for Christ on this earth because we have been so focused on getting them to heaven. This is a good, well taken point. But, this leads to what I think is a very unfortunate statement in the chapter "History and the Meaning of the Disciplines" in the sub-section called "The Continuing Error." He says that we have replaced salvation, which he defines as a new way of life, with one of its effects, or components, which is forgiveness of sins. This is a glaring error, since salvation is at its essence the forgiveness of sins. In fact, the new way of life is an effect of salvation, or the forgiveness of sins, not vice versa. Willard's statement is pure Romanism, or works righteousness. Because of his soundness in other areas I choose to believe that this was either carelessness in wording or that I may not fully understand what he is getting at here. Still, though it is one sentence, it is crucial in the scheme of things. It is forgiveness of sins that makes possible all that Willard talks about in this book in the realm of spiritual disciplines.
All in all I would say this is a book that should be read by the Christian as long as you balance it with some of the other books I mentioned. Willard's writing style is dense, he packs a lot of content into a few words. This is meaty stuff and well worth the effort.
