The kidsball works great. I have one in my CP, with a translucent blue replacement ball. Backspin probs are only in certain games, but are usually fixable thru the sensitivity settings in Mame. Missile Command works fine.
The biggest problem is how to mount it, given it's bloated, rounded-foo shape. I cut away a lot of the housing and put it into a square project enclosure so it's easier to mount. If I'd known how cheap the imperial balls were, I might not have gotten the Kye, but it all worked out in the end.