I also like trackballs although I prefer Logitech's trackman thumb trackball.

I also gave the Kensinton Expert a try but I really disliked it. For one I think I just prefer thumb trackballs, but beyond that I found the build quality really shoddy for that price. In particular the scroll ring is wobbly and rattles quite a lot on my model, it feels very cheap and is unpleasant to use.