the cautions that i have read with diamond sharpeners is not to press too hard, to just let the diamonds do the cutting. if you press too hard, you risk ripping the diamonds out of the matrix.
i use sandpaper/mousepad on a block of wood. it would last a while, but i could see the sandpaper getting worn out within a month or two (average time between replacement). but even worn out sandpaper is still good for sharpening...just at a finer grit than it is rated for fresh.