Probably overall using a gps to track speed and distance on a bike would all hinge on how many waypoints it measured....it will probably not pick up the extra distance you add to miss a pothole for instance nearly as well as a cyclecomputer will.....the computer tracks every wheel revolution.
But the aps have a lot of nice features esp IMHO (not that I have done it yet) for when more than 1 rider is in a group, and they may become separated, looks to me like cyclemeter would let me easily find a friend or them to find me as long as the email was setup right and the phones had signal :-).