Heinlein invention validated on Shark Tank

I was watching a taped episode of Shark Tank last night, and two guys came in with an invention: a gyroscopically stabilized unicycle.  You could rise it up to 15 mph and you could not make it fall over.  This is exactly - exactly - what Robert Heinlein, 60 years ago, called a tumblebug in his story "The Roads Must Roll." It's a shame it took so long to invent: it looks like a lot of fun, simpler and cheaper than a Segway and needing only a space the size of the user's body. there have been prototypes for self-balancing unicycles before, but these guys look like they have gone beyond the interesting toy to the practical transport.