I have a Crayola trackball (by the way, it's EXACTLY the same thing as a Kidsball, just a different color) and I like it a lot! The ball that comes with it is nice and heavy with a good roll, but I replaced mine with a translucent ball. Feels exactly like an arcade TB to me! I think you can get these for about $19 right now, I've seen them at Fry's. The best thing is it plugs right in and gets recognized as a standard MS mouse, so no interface.
On the other hand, someone has used mouse guts to interface an Imperial before, but I couldn't really tell you how they did it. Try a search of these boards, it was posted within the last 30 days or so...
Actually, come to think of is, doesn't the Imperial already have a PS2 interface?
