At the time I bought the book, I didn't have a set-in-stone pantheon, so I hoped to add many/most of their gods directly into my campaign world. I was told the information on using gods in your world would be useful in designing mine. I thought that the prestige classes could be used (without serial numbers), and that the holy warrior could be used directly.
I thought that the creation mythos would be useful, since at the time I didn't have one. I hoped the section on evil gods and cults would inspire me, giving me plot ideas even if I didn't take anything wholesale.
Generally, I bought this book because I heard good things about it and wanted to buy books that would help me make a pantheon and cosmology. All I can say is that two years later, I haven't used a single class, prestige class, item, backstory, spell, or monster from the BotR. I *almost* used one of their domains -- but ended up doing nothing more than typing it up and finding it didn't fit as closely as I wanted.