The Blue Moon Cult from Troll Gods is possibly one of my favourite cult write ups, and I've used a Blue Moon cult extensively in games. I like the random fact divination as a plot hook generator, I like the fact that secrets are a big part of her reason to exist. I liked the story of her birth in Wyrms Footnotes and used it as the basis of a heroquest to escape from the underworld when the heroes of my game all died going down Magasta's Pool as they accompanied the baby Thog to the underworld creche in the 2nd age.
The fact that everything is supposed to be secret is a great asset when no one really knows the Truth. I like the mangled translation of the phrase "Out of sight, out of mind" which I think is particularly applicable to the Blue Moon: "Invisible! Insane!".
Keith