I installed, and am happily using an ultimarc trackball. I can't imaging spending the money for a Happs, when the Ultimarc feels fine. It's all a matter of taste, and authentic arcade feel. Personally, I like the smaller footprint of the Ultimarc trackball, but then, you can buy a 2.25 tb from Happs too, it's just more expensive.
After it gets seasoned, it feels great (you have to break everything in a bit first)
I've been playing centipede like crazy!