I had some problems with my right hand at the start of this year, so I decided to remove a part of load from it. I started to use left-handed mouse and used it for several months (it's not always possible to work completely without mouse, especially in Windows).
Week ago I got a new mouse as present - Evoluent VerticalMouse 4 (for left hand), and used it for a week at home, while used old mouse at work. And I can say, that new mouse is much more comfortable for me - hand is comfortably placed on it, there is separate middle button (instead of less comfortable wheel click, although it also works). There are also additional buttons, that could be programmed to do something useful. Mouse works fine on all tested OSes (Mac, Linux).
It was so comfortable, so I ordered the same mouse for work and already replaced my old mouse with it.