I use one for 8+ hours a day. I keep reading about the design being uncomfortable, but definitely hasn't been the case for me.

I guess if I actually kept my hand directly in line with the mouse it'd be pretty painful. I just about always keep my hand in a slant, more similar to how you'd use a trackpad, or as if you were holding a sort of slanted mouse.

I've stuck with it because of the well implemented 2d scrolling. Using a physical scrolling wheel feels off at this point.