I just saw the author do a talk at a Meetup, and it blew my mind. I've seen a lot of neat things in Scala libraries (Shapeless, Scalaz, Akka, etc.), but this is unique in how far it's gone in expressiveness for fairly everyday programming, to the point of being able to choose your APIs types using implicits. It was like watching sorcery. I wish I could share the same talk I saw, but there are a lot of videos out there [1]. I haven't watched any yet to recommend a specific one, so perhaps the more recent ones would be best.
[1] https://encrypted.google.com/search?hl=en&q=scala%20rapture#...