Very useful upgrade if you run a slower motor in your car. This gives you the option to change the final drive ratio a lot give lots more top speed. It's easy to install, but it's a pain to get to it, you have to disassemble most of the driveline to change. I've done it so many times so it doesn't take me long to do now.
The quality is good and it doesn't seem to wear very fast at all. Using this will let you use up to a 22 tooth pinion, or even a 23 tooth pinion does fit, but the mesh is bordering on tight, but it is useable.