I don't use it regularly, but did for calibrating my SG and checking if my ODO was off in the car, it was 99.8 miles when the GPS read 100, close enough for me. The speedo was off by 4 mph at 60. The gauge read 64 when the GPS was at 60.
I also use it boating and occasionally on trips when I am not sure where I am going.
Garmin has saved my arse many times over the last couple of years. I had an i3 (stolen -- RIP Edith-I), then I got an i5: Edith II. I do drive rental cars more than my own, so I really count on the actual MPH reading and its Nav accuracy (better some times than others). It seems to be more accurate than Mapquest and Yahoo -- especially if you keep the updates downloaded/installed.
Tricky stuff is when the address number does not match the satelite map or the actual location - then it becomes a problem. Apparently when I recorded my friends driveway as his location I could not navigate to it with the TomTom anymore because it was not on a road. Also his actual address was 55 but according to the GPS and Google Maps more like 99 on his street. My friends farm in Maine was simular - the actual location was a higher street number on google.