I just had a "lesson" on dressage trainer Ruth Poulsen's version, "Rocky", last week. She uses him to help people gain strength, correct their positional faults, and overcome fear. (She's also rehabbing from a shoulder surgery and using him for that as she cannot ride "real" horses right now.) While I was nearly in tears over how terrible my position was, I could see how useful "Rocky" could be for building core strength etc. "Rocky" is not motorized, so all the movement comes from the rider, but once you get him going, he goes on his own for quite a while.
If I had the money, I'd consider getting one for myself.