I have a Metroboard, and definitely feel less safe on it than I would on a bike at the same speed. The big problem is the relatively short wheelbase -- it's not hard to go over the front of the thing when braking. The Metroboard controller is pretty smart and applies braking force gradually, to give you time to lean back -- but that means stopping distances are longer.

On the whole, I thought it a rather wooly way to go 2 miles from my house to the Caltrain. For trips around 1 mile or less I think it would make a lot of sense.