When using the Naos 8200 mouse I got excruciating hand pain which only got worse and worse as I continued using it (hoping I would get used to it). When I realised that the pain won't go away, I exchanged Naos for an Avior 7000, and that model, while not giving me outright pain, is not comfortable either. The problem is bad enough that I'm already buying another mouse of a different brand.
I don't know, maybe the fault is with me, but I have to say that I have an average-sized hand (female) and that I never had a problem with ergonomics of any mouse before. I used Logitech MX Revolution, Razer Abyssus and several cheaper mice in the past without problems.
I give it two stars because apart from this, the mouse seems solid with lots of options for customisation.