I use a GPS when traveling in an area that's new to me or when I'm leading a group. I've used a GPS in the past to know my speed but my F6B speedo is correct. This is my eighth MC and the first with an accurate speedometer.
Have used a GPS for several years. Mine is a Garmin 450. no voice. Really helps on long trips. Also agree with Steve. Sometimes down here in the back country it helps to find my way home after a long ride.