I'm rating this a 5 mostly due to the personal significance I derived while reading, as well as for the enormous theological insight Bonhoeffer's book gave me, in particular about what the Beatitudes actually mean. I put this in the 'life-changing' heading of my personal library, and I'd estimate that about 25% of my copy is now underlined. As for readability, it was a bit of a slog at points as Bonhoeffer's style is quite academic, though his insights are broadly relevant. Overall I found his explanatory section on the Sermon on the Mount to be the highlight of the book. I highly recommend the book to anyone willing to put in some work to derive significant insight from the book. I find it so much more revealing that not only did Bonhoeffer preach self-sacrifice and complete obedience to Christ, he actually lived it to the point that he re-entered Nazi Germany and preached openly against Hitler's regime and was eventually martyred just before the end of WWII.

A few of my favorite quotes:

On 'cheap grace' (the grace we seek without seeking repentance): "In such a Church the world finds a cheap covering for its sins; no contrition is required, still less any real desire to be delivered from sin."

On costly grace (the grace given by God, the result of which is following Christ): "Such grace is costly because it calls us to follow and it is grace because it calls us to follow Jesus Christ. It is costly because it costs a man his life, and it is grace because it gives a man the only true life."

"When Christ calls a man, he bids him come and die."

"The Incarnation is the ultimate reason why the service of God cannot be divorced from the service of man."

"If when we judged others, our real motive was to destroy evil, we should look for evil where it is certain to be found, and that is in our own hearts."

On the image of Christ: "Rather, it is the image of one who enters a world of sin and death, who takes upon himself all the sorrows of humanity...the Man born to poverty, the friend of publicans and sinners, the Man of sorrows, rejected of man and forsaken of God. Here is God made man, here is man in the new image of God."

"It is only because he became like us that we can become like him."

And about a hundred more!