Widely read blogger and book reviewer Tim Challies writes:

I’ve made no secret of the fact that I enjoyed reading Eric Metaxas’ biography of Dietrich Bonhoeffer. Actually, it’s one of my all-time favorite biographies; it’s readable, engaging and it deals with a fascinating part of history. But lately I’ve come across a few articles by experts in Bonhoeffer who say that it’s just plain wrong -- it’s a portrayal of the man that is geared toward evangelicals and, in seeking to make the reader happy, it succumbs to all sorts of errors.

Richard Weikart (California State University) and Clifford Green ("executive director of Dietrich Bonhoeffer Works") are two of the scholars Challies cites. 

Challies and others might also want to take a look this review by Victoria Barnett. Barnett, identified as "general editor, Dietrich Bonhoeffer Works, English edition," gets directly to the point in her opening sentence: "This is a badly flawed book."  

A cautious Challies says he "did not want to believe what those authors . . . are saying about Metaxas and his biography. But I am inclined to believe them as they bring the weight of scholarship and experience. They may well be right in suggesting that Metaxas got in over his head; and they may be right in suggesting that the true Bonhoeffer was simply too unorthodox to appeal to the likes of me -- the kind of person who read, enjoyed and enthusiastically recommended the book."

Challies concludes, "Sometimes the truth just doesn’t sell as well as a half-truth. And I’m afraid that we evangelicals may just prefer a safe and friendly character over an accurate one."