Currently use QC to make custom layers for Bonix TV - a vision mixing kind of application for OS X with a kind of QC stack of layers, each layer being a composition with published ports and some protocols for data types. Also generate sports graphics and twitter feeds and so on for video score boards at large sporting venues.
Spent a few years learning Quartz Composer as my way back into coding from a decade away doing design related work. Film, graphics, studied Architecture for a few years and got involved in publishing then film. Now moving into front side web development. Hope to get into app dev with Vuo and rapid app prototyping.
I'll try and come back to this and help. I started learning C to write nodes for Vuo but have been distracted. I would definitely start with stateless nodes to get your confidence up first, queue/hold the output if need be.