I'm one of those folks that had to have a spinal fusion surgery due to an injury at work. One of the side effects of it I have found is my balance now is HORRIBLE. I can be standing at the kitchen sink, doing dishes, and nearly fall on my side because my body suddenly decides which way is up. As cool, or helpful, or intriguing as any of these machines might be, I know for one, the day I sat on one, I'd put my head through a damned wall because it interpreted me trying to regain my balance and I'd wind up getting bucked off, even with such a low speed on that thing (Murphy hates me).
Not to mention, that thing would anger my back worse than sitting on the floor does already.