I have a 7 year old model, one of the first to come out. It is priceless. While camping we would cruise the back roads of wisconsin after dark with no fear of getting lost. Back then they did not do much street to street navigation. But newer models look great. You should be able to send routes to a memory card and give to each car at the beginning of each day.
But GPS or any other dashboard device are the number one items stolen from cars. So get them insured.
Also the cost of lost time for getting lost once or twice easily offsets the pruchase price.
Mikeeee