I play-tested an early draft of these rules with Jamis a couple years ago and I think the final version is an excellent framework. It provides a simple and comprehensive system for generating mystery plots. There are clear guidelines for crafting the initial hook, generating clues, obstacles, threats, and confirming hypotheses. There are expanded tables with a lot of highly specific prompts that are still broad enough for most genres. Most tables also have compact versions with prompts that are even more freeform.
I especially like the mechanisms for naturally escalating tension as you gather clues during an investigation.
This framework also seems pretty modular and easy to add to any game. You could start or stop using the system in the middle of a session without a lot of fiddling.