I made the switch to left-handed mousing after my right hand started to hurt when I went to sleep at night. The transition only took me a couple days. Maybe I'm special. I do most things with my left hand, but I write with my right hand, so that might have made the transition easier for me.
Having the mouse on the left side frees up the right hand for taking notes and using the key pad for navigating. When I have to use a right side mouse like with a co-workers computer, I have no trouble switching back to the right hand.
My left-handed dad taught me to play sports and my right-handed mom taught me the domestic things, like writing and brushing my teeth. I'm not ambidextrous, but I am a little left-hand dominant. So the key for a seamless switch to left-handed mousing is having parents that have different dominant handiness.
The real key is getting out of the typical mouse game altogether, like most geekhackers probably know. Personally, I'm a trackpad man, but if I ever tried the roller mouse, I might begin to favor that.