I picked up 5 of the original I-Balls during the Fry's selloff.
The Pro has faster polling, but is the same size, shape and color.
I haven't mounted any of them yet, but I have hooked them up to test them.
My biggest complaint about them is that they have the "computer pointing device" feel to them, rather than the "arcade controller" feel.
What I mean by that is that they don't "free roll" like an arcade trackball will.
When you stop rolling them, they stop dead, and they have more resistance than an arcade trackball would.
I've also experienced backspin on them trying to do really fast rolls.
I don't know if the faster polling on the Pro will minimize this; but it is definitely something to consider before doing a permanent mount on one into your cabinet.
As it sits, I am probably going to hack one into my cabinet as a temporary measure until I get a "real" one to replace it.
If I do, I'll let you know how it works.