The Microsoft Trackball (not Explorer) has to be the most comfortable and natural feeling mice I've used. Thankfully, I still have a couple. The Logitech version was good too. Since they don't make them anymore the next best thing is the Logitech M570 Wireless Trackball. The thumb trackballs are much more ergonomic and comfortable to use than the ones where you have to move your entire hand.
...except I got pains from clicking and scrolling with my index finger—on a thumb trackball (and a mouse, I rotated them, but there was no difference in this regard).
Thank gods for ambidextrous trackballs and thumb-operated buttons.
Most of the hand pain I've experience was due to constant left-clicking and scrolling on a regular mouse. Entire hand movement (found trackballs such as the Slimblade) has never bothered me. I think the thumb-operated buttons greatly reduce pain associated with the pain I was having.
I agree with this sentiment. Another benefit of the Slimblade is you can basically use your palm for the lower buttons.
But I preferred the Rollermice out of all the ergonomic devices that I have used for it's ease and flexibility. However, the Rollermouse cannot be used with most ergonomic boards because of the keyboards split, floating design or height or thumb keys getting in the way.
On another note, I have found most of my pain is caused by the mouse, so I can get by with a typical flat buckling spring or Topre board with an ergonomic mouse and feel fine. I just love ergo boards so much that I keep them around. And next to me while I sleep. And on long car rides. And when I go the grocery store. And....ok I'll stop now.