I have a d4. It was my first electronic drumset. I made the pads/hardware out of radio shack piezo buzzers, particle board, drywall screws and several layers of thick industrial tractor innertube. It worked good for practicing and at one point I midi'd it but they were not enough good ride cymbals that I could swing with and the attack on snares was such that I could never play a buzz roll. Also the Hi-Hat is open or closed
unlike the roland stuff or the Yamaha stuff where your can have multiple adjustments to the hi-hat to play half open. D4's are cheap now though.