I thought it wasn't a bad introduction to what the canvas is/does (rather than how you use it).

I just bought it a week ago. It makes canvas a lot more human. I've used it to create a few minor UI elements even, but main issue I've had simply the lack of rotate (or my ability to find it).

Still, its best tool I've used for Canvas.

http://soulwire.github.io/sketch.js/