Dunn was one of the most important and insightful Paul scholars of the late 20th century, and as most Paul scholars of that era wrote a "big book on Paul," this is Dunn's contribution to that trend. Making use of 40 years of research, this book is of course loaded with important insights, and deals well with nearly every important discussion on Pauline research over the past century or so.
In my view, the main benefit of the book is the helpful way it frames Paul's approaches to theology. Most popular level preaching and interpretation of Paul is deeply flawed because it deals only with what Paul likely meant in the most universal application possible, but not all of Paul's writing should be read that way. Dunn rightly recognizes three important layers to consider when interpreting any passage in Paul's writing:
First, the level of Paul's continuity with his Jewish heritage.
Second, the places of innovation where Paul broke with his Jewish heritage.
Third, the ways that Paul's ideas and arguments were influenced by the audiences of his individual letters.
With these various layers in mind, Dunn rightly shows throughout this book that Paul was constantly in dialogue with his Jewish past, his conversion experience, his opponents, and his churches, and these dialogues shaped his letters in some surprising ways. Such layers and dialogues are not often enough taken into account, especially by popular preaching and teaching, and this has caused a lot of confusion about what Paul really meant.
The book is not without its problems, though these problems most often extend from limitations of "theologies" rather than from Dunn's treatment of the subject. Most prominent is that fact that theologies rarely attend closely enough to actual exegesis of Scripture passages, and this book is as guilty as most. Far too often I read one of Dunn's important conclusions and thought to myself, "But, how did you arrive there?" A book of this sort simply does not have the space to allow an author to lay out the internal logic of individual verses and sections of Paul's work, as theologies of this nature tend to focus more on "big picture" questions. It's a limitation I understand, but it still limits the book's value as a conversation partner.
On the whole, this is a great book, which I will revisit many times over the years. Dunn's conclusions are not always satisfying, but they are reasonable and well articulated. Most helpful of all is the way that Dunn, in the introduction to the book, lays out the extreme problems faced when writing a "theology," and so he approaches the task fully self aware that the entire enterprise is inherently flawed in certain regards. This is an indispensable book for any student of Paul.