I consider the BotR to be the finest third-party book published to date. It is far from perfection, however, but the total amount of usefulness within the covers of this book cannot be matched, in my no so humble opinion. I do plan to write a full review of this book soon for ENWorld.
But who should buy it? In my estimation, anyone that wished to be a Dungeon Master, running a campaign where religious themes are to play a major role. By this I mean where you have clergy that are more than just "Priests of the God of Fire" but have actual DOGMA to go with that God's beliefs. Now, obviously, if you have no set pantheon and wish to use the ones presented in the book, then the book will have the most value to you.
But I dare say that even if you have no intention of using the pantheon shown in the book, they serve as the best worked examples of complete, three-dimentional deities in role playing. Thus, using them as a template for the creation of your own pantheon, mining them for ideas to use in that new pantheon, and some additional rules for applying these ideas... it really does not get much better than this book.
Is it for everyone? No. No RPG book is. No book of any kind is.
Does it have a lot of crunchy bits? No, but this is not a downfall of the book at all. It is a wonderful testament to the system that such a pantheon can be handled in such detail without the need for a large amount of new rules to do it.
In the end, I cannot recomend this book enough. It rates a '5' from me.