I made a similar thing (also aptly named smiley slider), but based on SVG instead of canvas: http://imagine-it.org/svg/sliderplugin.html (https://github.com/pamelafox/smileyslider) We used that for an app for letting people rate how much they liked talks at Google I/O.

Since then, I've used the same slider codebase to make a colored slider (green -> red), and I use that in my nutrition tracking app for letting people rate how well they avoided food groups each day (screenshot: http://www.everyday.io/img/tour_logs.png).

I find it easier to drag a slider than pick a choice when it comes to assessments. It's less stress for the user. Or atleast that's my hope. :)