I've had one of these RAT7 mice for a month or two now, and so far the build quality is very nice. The different pieces screw in with a little hex driver that actually lives in the bottom of the mouse, so you never have to go find it. There's a nice sensitivity rocker button to turn up or down the mouse sensitivity, the main scroll wheel is metal with a nice textured rubber grip band around it. Haven't used the thumb wheel much, and the big red button seems to be able to only be programmed as a modifier button (like to change the function of the other buttons), not as a regular key in of itself.
Overall, I'm greatly satisfied with my purchase.