This is one of the slickest implementation of a sketching app I've seen. The API is nice and the controls are self-explanatory. I've been working on a drawing app for the past few months (journeyship.com) and I'm considering replacing the main canvas with a sketching pad like this one.
For those of you who are curious about other projects out there that offer similar functionality, take a look at these:
http://www.createjs.com/#!/EaselJS/demos/drawing
http://fabricjs.com/freedrawing/