Using Processing as an entry point in an Introductory Course
I am currently designing an intro to programming course for middle school students using Processing. In particular, I am using these two resources to focus the curriculum:

Make's Getting Started with Processing: A Hands-On Introduction to Making Interactive Graphics

Raspberry Pi Foundation's "Introduction to Processing" Resource

Make's resource moves all the way from simple, one-line programs like ellipse(50, 50, 80, 80); to OOP, file IO, JSON, data visualization, etc. 

To supplement these resources, I am using videos from Daniel Shiffman's YouTube channel The Coding Train. Of particular value are his short coding challenges where he completes a programming task in the allotted time while talking through the design process.

I find that Processing's power comes from its scale combined with its ease of entry. I can confidently say that students will be writing "real" code on day zero and getting instant results on something other than the command line. Plus, there's the satisfaction of starting to learn programming with a text-based language instead of something like Scratch which can feel restricting and elementary to some students. And, as Daniel Shiffman's videos show, there's really no stopping how far you can go with it.