Yeah, the side to side, or east-west axis in their terms, looks like a tough one. The basic idea makes sense, you perform only one movement on a normal keyboard (pushing), on this contraption each finger has 5 motions. Just that alone means an 80% reduction in how many times you execute a single motion while typing.
If one of these were available for a lower price, I might play with it, but I don't see spending over $500 on something that doesn't have spare parts available if it breaks!