As a new smartphone user, I think I downloaded all the cycling related apps to try. After trying each one on a ride or two, I really prefer the MapMyRide app. Shows all the relevant data I need; distance covered, time, route ridden and average speed. I know there are fancier apps, but for my needs, MMR fits the bill quite nicely.

Of course as with any advice given here on Bike Forums, YMMV.