I used to feel pain and fatigue in my forearm in the evenings, after being on my computer at work all day then surfing thee web on my PB at night. I learned that I was suffering from a mild case of RSI, so I bought an Evoluent Vertical mouse. I use it on my Mac at work, and I no longer have pain in my forearm at night while surfing with either the PB trackpad or a BT mouse.
It looks a bit odd and seems waaay too big when you look at it, but it feels very natural and compact in the hand, and I was used to it in 2-3 days. It doesn't even bother me switching to other mice.