It's a great concept for folks who just want to see where they've been. For routing, a plane old map probably works just us well. The ability to see where you've been (especially if you ride with others) is great.

But ... I think these devices are much better for roads than mountain bikes. They calculate speed based on tracking net position changes. So you could ride around in a circle at 7mph. Hard turn are pretty frequent when you mountain bike. So are elevation changes which are really important to calculating distance travelled. Here again the GPS is at a distinct disadvantage. A hillside may have a bowl shape but all the GPS unit will see are the points you have actually been with a sharp joint at every measuring point.