Initially, I found the book a bit repetitious because Johnson uses a sermon format to work his way through Revelation. However, this proves to be useful in due course, as the regular reminders about aspects of what he's saying get reinforced by the repetition.
Beyond that this is an excellent exposition of a Bible book that's often held to be too difficult to understand. Though I've read it a few times, I've also avoided it more than other books of the Bible, because of that sense of difficulty.
Johnson's approach to the book shows how organised it really is, and how the symbols work together with other books of the Bible and aren't just things John the Apostle dreamed up. He's also very good at getting to grips with the numerology of the Book, showing how we ought not to treat the numbers as stats but as symbols too. This makes considerable sense of the 144,000 and 666.
In the seven letters to the churches section, which always seemed to me to be a different kind of book, he shows how it fits to the whole scheme, and he spends a lot of time on this section, going into the history of the various cities and how what is said about them is particularly relevant in each case.
All in all a valuable and useful commentary on Revelation, and one that I've already started to re-read.