Many, Many, Many different reason's..
"Hand's on", being the most important.
Ive got the MPD16.. And love it coupled with FL6, and NI Battery2. However, like some ppl say, the pad's have to be hit in the center [Cant be hit on corner's, atleast.], for it to register.
If your going to spend more then the $99 an MPD16 costs, Buy an MPD24, or Korg PADKontrol.