I've had one of these Logitech Trackman Marble FX trackballs for about eight years. Best pointing device I've ever owned ergonomically. The beauty of the design is that you don't twist your wrist. It's also one of the early opticals, so the only time you need to clean the rollers is when they physicially stop the ball from moving. The ball is nice and big, which gives you good control.
Sadly this one is now dying. The ball sensors are fine, but the right-click button is having trouble recording clicks and that's getting annoying.
So any suggestions for a trackball as good as this? As often happens with capitalism, this is a great product so it's been discontinued.