If you spend a lot of time listening to podcasts and reading countless pages of forums, the book will not be a revelation of any sort for you.
The book is a collection of the strongest (best supported) theories out there, and the authors dig through a lot of information to provide that supporting evidence. Their sources (all cited throughout the book) include information from interviews and appearances from Jo, as well as material from the HP books themselves.
I agree with K. Peterson's comments. As one who is a huge Harry Potter fan but who doesn't have the time and energy to read all those countless pages of forums, etc., I found this book to be a very handy summary of all the major theories and the various arguments pro and con for each. But I would agree that someone who has been closely following all of the above mentioned sources of information may not find it very informative. My verdict: this book is great, for the right audience.