I haven't tried building projects with it yet, only played the games and watched some videos. I recommend the following two videos:
First for educators check out:
Second for a great tutorial on how to create your own blocks:
The second is rather well done and the type of supporting documentation that students and teachers need.
Plus it also does Logo see the Turtle example and it can be run Offline!!!
P.S. I am having too much fun (mostly with Blockly and thinking about learning/teaching/doing programming. Oh I also made some more progress at Code School