Did you get the idea from the tennisball-keyboard controller that was posted ages ago?
Yes actually, though it was obvious that wouldn't work with all the extra keys getting pressed beneath the tennis balls.
Even with single button presses per pop-kun, I run into some issues with certain chords. For instance I can't hit left green and red at the same time, nor left white with both greens and right white. Sometimes nothing registers when I do an unsupported combination, and I lose ~5-10 bars ;[
I'm currently stuck around lvl27 songs because most of the hyper stuff has complicated chords that are impossible to play on this device. And trilling those chords adds so much more difficulty to songs that are already challenging me.
Sweeps can be really hard too, considering I need to hit the buttons -straight- down or they don't register. But a few hours playing lamento (normal) was enough to get used to that.
my pop'n controller 
