This is the best tool for physical computing. Kids and adults alike will love it. The 1.0 beta has been released today. Time to give it a try! And only if you've actually tried it, what do you think??
I know several teachers who use MicroBlocks. They praise its simplicity, the ease with which kids get started coding, and the high level of student engagement they see. One teacher I know uses it with students as young as grade 3.