I am another one who finds the idea interesting, but doesn't need it. But I have few thoughts on it anyway:
* I would probably rather use a layer on an existing keyboard.
* It's quite tall (3*6 for just digits), which requires quite much of hand movement. I would consider another alternatives. Maybe 4*4 (which is still quite a lot finger movement), 5*3+thumb (a bit ErgoDox-like) or maybe even dual hand 2*2*4.
* Where would you put keycaps with bumps?