The best thing about having a GPS is you're able to assign a number to everything: Your turn is .5 miles away. You will arrive at 3:40pm. You are averaging this speed.
I took the GPS with me on a bike ride while getting some stuff done around town today (hence the 18 minute stop time) and it was pretty great. I think I ran a pretty respectable average speed given that my chain bound up and rendered by bike unpedalable 4.4 miles into my 6 mile ride. Luckily for me, I was done with the uphill part of the ride and actually set my 19.5 mph max speed while unable to pedal.