For me it's the best device I ever had to work with.
It's a 5 Button mouse with 2 for the Thumb, and three on top.
The scrollwheel can easily be swithched from stepping mode to free wheeling which is very convenient when surfing the net or reading big documents. It also supports sideways scrolling. The middle button is behnd the wheel. I also have come across mice with scroll wheels also been used as middle button and found this setup almost unusable.
As I hold the mous loosely between thump and pinky, I have three fingers resting relaxed on top of mouse and use the middle finger for scrolling and pasting.
Early last year I had to work at a customers premise with a 2 Button mouse with tail
After changing to use my own - and of course some treatment and changing to my left hand for mouse usage - I'm now fine again.
BTW: There I have to work in a W7 environment