How I Use It
I've used this mainly in a coding club setting, but my students started asking to use it in class. I would allow my advanced, and early finishers a chance to solve puzzles. I would not recommend allowing students to just solve puzzles, however. The code.org curriculum comes with simple 'unpluggled' activities, that introduce skills. Be sure to follow the curriculum to prime your students with the real world, hands on application before they begin solving puzzles. This will greatly improve their experience and allow them to work through the more challenging puzzle problems.
Code.org has really improved from their initial product (which came out last year). I recently attended one of the first trainings in Texas, which I would recommend. They've added more courses--scaffolded by ability/grade level--and standards alignment documents. I plan on using whole-group it in my upcoming graphing lessons. It will serve as a great hook and extension in the classroom.
In my technology club it is a great teaching tool for creation. We're going through the course and using storyboards to create vocabulary stories to share with the school.