I have used mapmyride and google's mytracks. if you do a road ride, or an out and back ride you can easily see where things don't match up exactly. For that reason I am a bit leary of things like top speed (yeah, I guess I could have been going 41.6mph last time I went mountain biking, but 35 is more likely). GPS often has a default target range of with 20 meters. Good enough for government work...

Interestingly using it in my car, the GPS ap is a lot more accurate than my car's speedometer on the highway (60mph on gps = 1 mile in 60 seconds, 60mph on speedometer = 1 mile in 65 seconds).