Just finished the book. Having read some N.T. Wright before, the style is familiar: thorough, sometimes with British humour. The book is a response in an ongoing debate between John Piper and him.
If you're interested in reading Paul on this topic, I think this book is worth spending some serious time on. However, if you feel that the Reformation gave the final interpretation of the concept of Justification for the rest of eternity, don't bother. The book would be a waste of time for you.
The key difference, as I understand it, is that Wright does not believe that Paul anywhere teaches that some quantity of moral merit is being imputed to a Christian, but rather the death and resurrection of Christ. And he takes some 250 pages to explain why.