I regularly use a GPS on known routes just to see how it does. I don't have a problem with using GPS for nav, but I don't use just the in-car receiver, I also have the software on my PC with the same map/DB as the receiver.
I use the map software so that I'll know the route before I get in the car, then I let the GPS follow along as I drive. Mine says "recalculating" quite often
But that's one of the Garmin Nuvi receivers. For backcountry driving, a GPS 60CSx with road and topo maps is a much better option. Having the software on a PC so you can get a better picture of what the GPS is doing helps a lot.
To take that many people with children that far off road following a GPS blindly is a learning experience.