I bought this mouse to see if it would be easier on my hand than my current trackball mouse, since my fingers get pretty tight and achy at the end of the workday. I had high hopes for it, as the position of your hand is definitely more ergonomic, but it did not work for the kind of work that I do, which entails clicking accurately in documents (and a lot). This mouse is not very precise at all, even after messing with settings, and I would end up contracting my hand on it like crazy trying to get it to click where I needed it to. Counterproductive since then my hand was all contracted AND I would lose so much time over a day having to do this every time.
If you don't use this mouse for work and/or your work doesn't demand a lot of precise clicking, this could be a great option though. Very comfortable if you don't need it to be precise/fast.