Short review: The basic idea is right, that anything added to the gospel means it is not the gospel as Christ has presented it to us. The problem is that in his attempt to counter legalism, his removes most of scripture and most of the historical teaching of the church. Anything in the Old Testament (according to Farley) cannot teach us anything except what we were saved from. Most of Jesus' teaching is discounted because Farley says he was teaching to Jews that were bound to the old covenant. Even Paul's teachings are sometimes discounted because Farley says that anything that disagrees with his point was referring to Paul's life before he was saved.
A longer review on my blog at http://bookwi.se/naked-gospel-farley/