I've just finished Exploring JRR Tolkien's The Hobbit, and I must say that I was glad that I got it. I suspected very much that I would enjoy this book after listening to lectures about The Hobbit on the Tolkien Prof's website (as well as having listened to the classes that he taught and uploaded to iTunes).
What you get with this book is a chapter-by-chapter examination of The Hobbit, with the addition of various information that Professor Olsen has that can be used to make educated guesses about some things where only a guess at a meaning can be made. There were times when something was mentioned, and I was able to get a clearer idea of something that was said (or realize that I had made the wrong assumption about something altogether).
All-in-all, I'd say that it's worth buying whether you are a fan, or only have a passing interest in Middle-earth.