I have the Speedohealer, from what I have read about the SpeedoDRD on the forums, I should have bought it and saved about $40, it seems to be just as good.
Adjusting the speedohealer is easy. Getting the exact correction is the hardest task. Use a GPS to verify your calibration. I first put in a -4% correction, then kept increasing it until my Speedometer matched my GPS exactly. I ended up with -8% correction (with stock gearing).
Now that I have installed the 16 tooth gear, and a brand new set of Avon Storm II's, I have a -13% correction.