I have done a great deal of work in Processing, and have taught a few classes & workshops on the subject.
Processing is often my first choice when prototyping ideas that have a significant visual component. The first version of my Musical Chess engine was written in Processing (I eventually ported it to Java). The first version of the Whitney Music Box was in Processing (the web version is in Flash), and the first version of my Slitherlink Puzzle generator was in Processing (I ported it to C++ / Cairo).
More recently I've also done a bit of work in Cinder, and in Open Frameworks.