I wouldn't ride a Segway.
My grad school research subject was a small robot which had very similar dynamics. You have an inherently unstable system, which you use sensors and a computer and electric motors (and some fancy math) to stabilize. It's really cool when it works. But if the controller tuning was off slightly, or if something went wrong with the motor or drivetrain, or something else unusual happened, the robot's attempts to stabilize itself could make it flail around like crazy, fast and hard enough to break your arm if you got in the way.
Therefore we always used a dead man switch, which you could release if something bad happened, and the computer would stop trying to stabilize the robot. Of course when you released the switch, the thing would just flop over...
I don't see any reason that the Segway would not be subject to the same problems. When it gets into one of these bad situations it is either going to behave badly, or the controller will be smart enough to realize it is in trouble and it will stop trying to stabilize. Either option seems likely to result in the rider doing a face plant.
I'll stick to my bicycle with good old manual/gyroscopic stabilization, thank you very much.
