I travel extensively, in a different state every couple months, I currently use a Nuvi myself. As stated before the units need to be updated from time to time, but overall your major highways/roads stay the same. Bussinesses and such change locations, but overall they are very handy when hitting a new area for finding locations such as Lodging,post offices,FedEx, restuarants and the local Hooters.
I use mine in conjunction with a road atlas. When hauling a 36ft gooseneck toolbox it comes in handy knowing your exiting to the left/right in 0.7 miles when you have 4 lanes of traffic to navigate in Houston/LA/KC.
Money well spent if you travel alot.