I had one on my RS. Its a nice setup. Fairly simple to wire with vampire taps. Easy to remove with little to no damage to the harness.
It is basically an LED display that reflects off the windshield. Someone had a picture of it on my car, but now I can't find it. I had just pushed the last max speed button and it was displaying 132mph.
It can also display RPM instead of speed. There are also a line of LEDs that can display speed or RPM as a fuction of 1 led = 1000 rpm or 10mph. So in a sense its dual display. Very nice if you have trouble sticking your speed while crusing. Also a cool wow factor at meets.
I stuck the controller, buttons sticking out, in the slot for the coin change in the RS center console. That way I could control options like brightness and which mode I wanted to run it in or just shut it off. You have to adjust the angle on the mount for best viewing. The bit of tint is just that a square of tint that helps you see the numbers when in bright light. The unit functions best under darkness or cloudy days.
The blue led model is best as the red led model can get you pulled over.