At the end of the day the success of the application is going to be dependent on the accuracy of the gps positioning, the application functionality is fine.
I wonder if some people for reasons I can't explain simply don't get a very good position provided from their iphone or android, and everything which follows is compromised.
My own experience is that it's accurate to within a few metres, and recognises dog legs in cycle tracks, and slight deviations from the road very accurately, so it's not just tracking the road.