If you lead trips, GPS is nearly a must. Larger screen and the ability to plan your route. Many times you will receive the wrath of a pack mentality should you find your self heading for a dead end or stumble across gravel rather than asphalt. I tried using my iPhone as a GPS tool on a trip and found that the screen was just undersized enough to warn me of the next turn late enough to throw everyone behind me into hard braking. It was after THAT ride I settled on a Zumo 660. Love the feel of confidence with others behind me, it's one less thing to worry about