This book was very provocative (in a great way). I believe the act of preaching is, as Campbell argues, an important embodiment of the Church's non-violent resistance to the forces of evil in the world, and I also believe preachers should be aware of that!
On the minus side, some of the terminology the author uses, mostly picked up from Wink or Stringfellow, starts sounding a little clunky or even outdated (à la 1984 or some such cold war-era dystopian novel). I think the idea is to avoid the Pauline terminologies that have probably become cliched for most people, but I don't know if it works that well. A minor annoyance, however.
The other thing I sighed at every once in awhile was mention of issues on which today's Christian community is very much divided, especially sexuality. It seems to me that partisans in issues such as these are mostly speaking (or screaming) past each other, rather than attempting a genuine understanding of differing points of view. Campbell doesn't come off as a partisan, and the point of the book is not to rehash this or that issue, but when he does touch on these topics, it feels like he is pretending they are largely settled, and they are not.
The reason why I wrote so much about that last concern is that this reality of divisive issues must be taken into account when we are talking about the Church's witness before the powers. I also am generally frustrated with the state of dialog in the Western church today, and acting like no more needs be undertaken is disingenuous and perhaps even an abuse of power itself.