I am one the few with the latest and final revision of a Marble+ trackball. This is a picture of my own Logitech, some hours ago, next to a natural orange juice :)
Logitech stopped selling the best trackball in the world! The Logitech Marble+ did not have enough success, relative to index-finger-controlled-trackballs, and it was discontinued. What a shame!
The Marble+ is a thumb-controlled-trackball with optical accuracy. It tracks a ball painted with a special red & black coat and it features a scrollbar control, 3 keys operation, and a superb anatomic design, making it the best pointing device this planet has seen... yet.
Unfortunately, people tend to buy crappy alternatives, such as ALL index operated devices. The thumb is our most versatile finger; it would be more natural for us to use the thumb as the control finger, but because of the immediate association between a pointing arrow and a pointing index, people go for the easy option and opt for such [much inferior] devices. In the long run, people develop some kind of minor RSI injuries, precisely because they go the wrong way, using "typical" devices instead of the "natural" alternatives.
Microsoft also replaced its original "NATURAL KEYBOARD" for a smaller, not-so-good "NATURAL KEYBOARD ELITE"... Consumers are to blame, since they often answered they would prefer the original to be "smaller"... Health has a price and to few are willing to pay for it.