I'll add my opinion, since everyone else is.
My experience is only with the PC-FX controller, although I understand it's identical to the Duo-RX controller.
My feelings lie somewhere in between; I feel the controller is neither spectacular nor crap.
Contrary to Tatsujin, I think it feels very comfortable hold and this is it's biggest advantage. The six buttons feel fine not being concave. But like Mr. Tiger, I believe the biggest issue is with the D-pad which just doesn't feel quite right somehow. Not something that makes you want to commit suicide, like the D-pad on the "Battle Pad", but not warm and squishy like... well, nevermind.
The NEC Avenue 6 button pad is a better controller, all around.