I first have to say that I did not buy this so much for the business angle--I'm more just on the lookout for any new Buddhist material I can find. This is probably the freshest book in the realm of "American Buddhism" that I've read in the last year. I hadn't heard of Roach before, but I now feel that he truly is one of the most important Buddhist teachers in the U.S. today. (If you haven't already seen it you should look at the interview Amazon did with him.) The writing in Diamond Cutter isn't always so great, but Roach has a superior talent for explaining elements of Tibetan Buddhism in a way that is light years ahead of many other books out there in terms of accessibility. Perhaps that's because he is American born, so he really knows how to communicate with his contemporaries, but even beyond that he appears to have a great mind. This isn't great as a Buddhist primer, but if you are at all familiar with Buddhism you'll probably get some real insights out of this one.
This book presents many insights on parts of the Diamond Sutra. (The "Diamond Cutter" name is interestingly new to me, as I have been only familiar with the shorter "Diamond" and the longer "Diamond That Cuts Through Delusions".) The author's interpretations are refreshing, even without the context of business challenges, of which his diamond venture serves as a unique background. It is rare to have aspects of the Dharma presented by a young, life-engaging monk, and an American to boost. In particular, the notion of "mental imprints" is deeply important with respect to how one perceives and constructs the world. As a bonus, I learn a few things about diamond, diamond cutting, and the diamond trade.
Unlike another reviewer, I am supportive of Geshe Roach in his presenting the Diamond Sutra through the particular form of this book -- a manifestation of what Buddhist tradition calls "skillful means".
About the 4 stars: I wish the writing could be tightened up in various places. (But then again, this subjective observation of mine may be the effects of imprints left by my earlier writing courses.)