Really, QC was one of the cooler things Apple has done since HyperCard.

I wrote a video editor that allowed you to use it to build plugins with it: https://vimeo.com/121663242

Unfortunately, life got in the way of finishing it.

I think Apple bought it and renamed it to Quartz Composer. I remember you could use it to make screen savers and could even write your own patches for it in Xcode.