Just got ahold of a HW6500 and I found that if you roll the control pad across a diagonal, it will not always generate a key up code for up and down.
I verified with a few different games.
The only fix for that is to force button up's for anything other that the new button down - thus destroying the capability to do diagonals.
Unfortunate, since the mini-joystick feels pretty good.
Anyone have any other ideas?