The TypeMatrix keyboard. The "backspace in the center" concept seems like a stroke of genius. It isn't.
Reach for 'T' with your right hand, or 'Y' with your left. That's the motion for backspace. A traditional BS requires only a slight reach or twist of the wrist (depending on if you BS with pinky or ring finger), but on the TypeMatrix it requires an extension of the entire hand to reach.
By the end of the first day, my hands were sore, worse on the 2nd, and on the 3rd day they were in pain by noon. The "ergonomic" TypeMatrix... isn't!