After 2 days of acclimation time I can say that the mouse works very well and also very good in the hand. Although I had never pain in the hands, but nevertheless the Ergo Mouse feels simply more comfortable than a regular mouse.
The keys are smooth and the functions are exactly the way you look at the expected standard mouse.
As regards the size: The mouse is actually something great but even that should not be a problem because the shape very well adapts to the hand (to clarify: I'm just 170cm tall and have relatively small hands).
My only criticism is that the mouse is too smooth. The special position of the hand and the wedge shape of the housing has any such constantly feel as if the mouse would slip forward out of hand. We'll see if that will change with prolonged use. If one were to make the material easy grip (roughened grip surfaces) instead so extremely smooth, which would probably also fixed.
It was also criticized that the mouse is easy to slip and thus with double clicks. I can not understand. While it is true that the mouse is very smooth and you might need a touch more precise order, but a problem that should - if at all - only represent extreme Grobmotoriker.
In the end I am happy that I have now decided on this shape of the mouse, even if it looks very chaotic at first glance. I would never want a normal mouse. For the price it is a really mature product and I am, however, very zufrieden.Wegen of something smooth surface I have to deduct one point. But what is still a very clear buy recommendation.