A few years ago, my friend Austin Sarner and I were writing a user friendly content publishing system in Objective-J. We designed a majority of the controls and UI components using canvas and found it incredibly fast when the proper caching techniques were used.
One of the components of the application I worked on was a text editor that resembled Apple's Pages software. The text editor was implemented entirely in canvas and would wrap live in response objects intersecting the text. Redrawing the text live was slow but we ended up finding a solution. We cached bitmaps of the characters, lines and paragraphs and only re-rendered areas of the canvas that changed.
The code was open sourced and put on GitHub, if anyone is interested:
https://github.com/austinsarner/Frappuccino/tree/master/Frap...
P.S. There's also a really fast iPhoto-style media browser in the repository that is implemented in canvas. It uses image caching techniques for animation.