SPEEDHUT instruments

Any experience with them? Are they worth it? I want to build my own dash in my 72 Dart. And after my 5speed conversion I really need to know my speed correctly. Calibration is not possible. And Speedhut is the only brand which can offer speedometers in KM/H (metric) up to 250 km/h. And they offer them with GPS sensor. No cable required. So it seems very easy to install and accurate reading/measuring.

Anybody here who already has them installed?