Has anyone used both? I currently have a MS explorer on my pc but I would like another trackball to use with my iBook. I have a fair amount of nerve damage in my hands, one result of which is that I cannot use a mouse for more than ~30 minutes before my hand goes completely numb. I also cannot use a thumb controlled mouse - my thumb quivers uncontrollably if I try to use the level of dexterity required for any small cursor movements. The MS is ok, but doesn't feel just right - even after a cleaning it still has a certain amount of drag. I really liked the Logitech Marble FX, but they've quit making them and eBay specials are $60 plus for used ones.