I once met a unicyclist on a bike path; he had already ridden about 5km and had another 15 to go. I think he was out to impress. Looks like he succeeded with me at least.
I wonder how road rules would apply? Where are you supposed to put the bell, lights, brakes etc etc. I suppose this would make it illegal to ride these in public places.
2nd thoughts, it falls outside the definition of "bicycle" which has 2 wheels.