I've used one Dakota Digital product. It was ok but obscenely overpriced.
Please understand there are differences between GPS and the speed sensor approach. First (and maybe foremost) is your speedometer must match the VSS pulse output of your transmission. Many of the aftermarket VSS speedometers accept any output VSS rate but have a specific method of calibrating so your speedometer will work with the size tires you have and the rear end ratio...otherwise it will never be accurate. Speedhut for example used to make you drive past a road marker, press a button, drive exactly 2 miles and press it again when you cross that 2 mile road marker. A timer inside the speedometer would understand 2 miles had been driven, measure the time spent and calibrate your speedometer to that. It wasn't perfect and it might take two or three tries. Some makers have you drive next to a vehicle driving a certain speed (maybe 60 mph) and then press the "set" button. All have some scheme and there's a bit of guesswork in all of them. The GPS on the other hand connects one wire to a known good ground, one wire to a 12v+ ignition switch "on" post, the high beam and dash lights and screw in the cable for the GPS antenna. And you are ready to go. Good luck!
