I just wrote door24 of my 2011 - Calender
I have written a lot of processing.js sketches and here are some of my findings
first of all - I like it very much - especially how easy it is to deploy on a website and the fact that on the client side only a browser is needed.
What I haven't tested so far are things like sound or video, or integrating libraries into the code. All the sketches in the advent calender use pure processing.
I thing this is a point where we can expect a lot of development in the next time, but where processing defintly has a clear margin at the moment.
So Processing and Processing.js will definitly coexist in my toolbox from now on :-)
processing.js advent calender - door 22
processing.js advent calender - door 21
processing.js advent calender - door 20
processing.js advent calender - door 19