Have you heard of One Wheel?

http://rideonewheel.com/

That looks absolutely amazing! The large wheel makes it a lot more usable imo.

But what's plan b when something goes wrong? On the electric skateboards you can at least break in the good old fashion way.

People have been riding skateboards/longboards without brakes for many years now. Plan "B" is often a good set of pads.

http://www.youtube.com/watch?v=b9yL5usLFgY

I rode an early version, it's pretty fantastic.

That's an expensive wheel!