I recently had the opportunity to try the Half Bike. I like the videos and the idea and it certainly looks like fun. What I learned with my few min. test, if I wanted to ride the Half Bike I would need several hours of practice, in my two attempts I was not able to ride it. The person who provided the opportunity could ride it but after practicing 15 min. a day for a week was still not comfortable.
I think it is a neat idea but from my short experience I believe people need to be ready for steeper than normal learning curve, it is not at all like riding a conventional bike.

Cheers, Bill