While I didn't need one, I've seen folks with them in their cars (built-in or temporary mount) and thought they are pretty cool.
So, as a toy, I picked up the Mio C220 Digi Walker a few weeks ago for well under $200 US.
It's handheld, or can be mounted off of the dashboard or front window, and tracks you on a map as you travel. It locates itself by interpolating certain satellite signals and the service is free.
I would say that travelling salesman or someone that does on-site support should certainly have one of these. Not only does it give you the route to your destination, but it also gives an estimated time of arrival, making it handy if you have to call someone and let them know when you'll be there.
Mostly, for me, it's just interesting and fun watching it track my progress around town and this part of Florida. The other day I was in a city that I don't visit frequently, and using the GPS's Points of Interest feature found out that there was a Sonic's drive Inn close by. Just had to have a cherry limeade.
It is truly a Space Age device.
