I guess I mostly just use it to compare one day to the next, not one rider to the next. And for that, it works. Plus it scares my wife, so that's fun!
I think even then it may be inaccurate. I think that at times it looses gps reception and that's when some of the stat keeping goes haywire. Sometimes I look over my GPS tracking it shows that I rode a straight line down a trail when I know I was deep in the trees in between two trails.
Accuracy probably depends on the terrain that you are riding.