Interesting book. Don't think I've ever read so a thorough discussion on the subject. Sanders delves deep, discusses several viewpoints and then offers his own and the reasons why. The book will definitely help you think about the subject and the various options in interpreting the relevant parts of Scriptures. The following quote is a representative conclusion.
Paul becomes most human when he encounters difficulty, and the sections which show Paul at his most human are 2 Cor. 11:16-29, where the problems are external, and Romans 7 and 9-11, where his Jewish and his Christian convictions come into conflict in his own mind. Once we see past the exegetical difficulties to the troubles of the man who wrote them, a moving picture emerges, one that is partly poignant and partly stirring. We see Paul the Jew and Paul the apostle of Christ, convinced that Cod's will is that he be both at once, and therefore never questioning their compatibility, but sometimes having more than a little difficulty reconciling his native convictions with those which he had received by revelation. He was a loyal member of the synagogue, but was flogged by his own people. He saw himself as helping to fulfill God's eternal plan, already announced in Genesis, but he was thereby pushing the Christian movement toward becoming a third entity. He knew that righteousness is only by faith in Christ, but, he still tried repeatedly to find a place for the law in God's plan. The most poignant point is the last one considered: he desperately sought a formula which would keep God's promises to Israel intact, while insisting on faith in Jesus Christ.