Finally got to do just a little testing with mine. It's great!
You may have heard me talking on podcast(s) about the arcade stick I created that can be either an Atari or Genesis controller. The Genesis compatibility feature was in anticipation of this adapter. It's really great playing arcade ports and/or arcade style games with a true arcade joystick on real Inty hardware!
I only tried a couple of games so far: Ms Pacman and Boulder Dash -- so no real putting the keypad through any paces yet. But I saw no issues at all. Just great control. No strange sounds or anything like that either.