I always thought those things were impossible to run in because the rotational inertia of the wheel is way higher than the inertia of one running in it. That requires you to run at almost constant speed (if your foot stays a bit longer on the ground, you do not decelerate the wheel much, but your feet ends up a few centimeters backwards. Repeat for two or three steps, and you lose your balance).

That changed when I saw the Ukrainian entry for this year's Eurovision contest (http://www.youtube.com/watch?v=h5DhH0jWyfk). On the other hand, that wheel is smaller and likely has way less rotational inertia than the ones one finds in the typical playground, and probably also has less rolling resistance. That guy must have trained, if only to make it look effortless, but it may be easier than one would think it is.