Maybe so, but I've been thinking about this since I bought a smartphone and started using GPS + apps.

I have a Cateye bike computer which I'll assume is the more accurate compared to GPS. For comparison, I've used both Strava and MapMyRide. By far, MMR is the more accurate, slightly more so in distance, and very much so in time. Strava's auto-pause is very unreliable for some reason. After a 1hr30min or so ride, MMR might be off time-wise by 30secs or so while Strava is off by as much as 2-3 minutes. The mileage is generally accurate to within .2-.3 tenths, which over a 20-25 mile ride I don't consider too poor.