The momentum of the scroll wheel is great. I hold mice by thumb and little finger and the G9x works really well like this. To be able to switch the scrollwheel from clicking to smooth with a button press is great!
The force required to depress the scrollwheel as the middle button is enormous. I'm a rock climber, and my middle finger aches after a few minutes use of the middle button. This seriously impairs the usability of the mouse.
An almost perfect mouse, ruined by an overly stiff middle button/scroll wheel. If the middle button/scroll wheel were better (softer), this mouse would get a 5 star rating (perfect).