I have found that I do not care for trackballs that are operated by my index finger. I use a logitech trackman (right handed) which uses the thumb to control the ball. When using one with my index finger, I find that my hand gets tried faster. Maybe if I tired using the middle finger it would work better, but I f find them awkward. You use your index finger for just about eveything, and have to take it off the positions ball to click or scroll.
The size of the unit looks ok, but it really doesn't look comfortable to me. As to reliability, by trackman is over 8 yrs old and still going strong. It needs a regular cleaning (like once a week or so), but it's still going strong. I'd really recommend you go someplace and try one for a bit in store before buying, unless the mail order place will let you return it if it doesn't feel good.
Its also incredibily funny watching someone used to a mouse try to use a trackball or tablet.
Reid