Very similar for me. My thumb/ring/pinky fingers are pretty much in the identical spot. The texture on the sides are also the same, but the wheel is a little harder to scroll though.The biggest difference is the sensor, it's much more accurate and consistent.
Well, looks like I won't be buying Logitech anymore. I found the Corsair M45 to be an ideal replacement for my old g9x. No marketing bluff, just an amazing optical sensor mouse with no tricks to up the dpi.
Size wise it's very good as well: