I recently bought the left-handed Elecom EX-6 trackerball. Why? For years I have used the Logitech M570 trackerball. I find that cursor movement is so much easier using a thumbwheel than anything else that I have found, and may well be saving me from mouse related RSI. However, I'm right-handed and have recently found that my right thumb is beginning to get a bit tired particularly when using the trackerball, so my solution was to buy a left-handed trackerball.
The Elecom is about the same size as the Logitech and they both seem to fit my hands ok (male hands, large but not very large), the Elecom ball does seem a bit less smooth - this may be because its new, having had several new trackerballs and replacement balls, they sometimes take a bit to bed in. The low-high setting buttons do seem to offer more than the Logitech but that can be done using the Logitech mouse settings. The real downside, I have not found a manual in English for the EX-6.
In summary, thumb wheel trackerballs are wonderful, wouldn't use anything else, but, thumbs can wear out just like wrists! Anyone know of a UK supplier that will sell me a replacement Logitech ball? - the places that I've bought from before seem to have disappeared.