My Take
This is a great tool to use to introduce coding concepts to students in a fun, game-like format. It focuses on the major concepts such as loops and conditionals, but the students use Blockly to write code, so they don't have to worry about the details of the coding language, which can frustrate beginners. The site also has great video tutorials, tons of support for teachers, and even a translate widget so that ENL students can participate with ease. The only con, in my opinion, is that students can complete all the courses without actually ever learning a specific coding language.
How I Use It
I use the hour of code as an introduction to our unit on coding. When the hour of code was completed, Students who showed aptitude and ability, and who had mastered the concepts, moved on to Khan Academy computer science modules, while students who struggled went into the "Beyond an Hour of Code" exercise, to delve deeper into the concepts without actually having to write code.