I was looking at some of those devices, and they are pricey. Here in Albuq, some of the stores had them at upwards of $1,000+.
That said, the free option, aka looking up driving directions on the Internet can be a pain. Main reason, I don't know who writes those directions, but some of them are flippin ridiculous. Case in point when I went up to Santa Fe to see Munich. I got lost and was driving around the city for about 45 minutes trying to follow their bozotic directions, and then stopping to ask for help.
When I finally found the thing, it was like 2 turns after I got off the highway, and none of that jazz the stupid Internet directions had me going. When I see this in Albuq (my home city), I know the area well enough to be able to say "that's stupid, just go here", or "well the traffic there isn't so good, so..."
However, when it comes to cities that I don't routinely go to, and are not my place of residence, I don't have familiarity with the area on my side.
All in all, I'm thinking it might be worth it, if one has the money. More pricey initially? Yes, but then again gas isn't exactly cheap (cheaper then it was 5 months ago, but still), and when one starts having to make multiple trips (aka doesn't find the theater until after the ticket booth closes, and the movie would be about half over anyhow), bleh...
As to maps, don't know about others, but trying to read a map and drive... One needs to be able to pay attention to their driving and the other cars on the road. One almost invariably has to plan the trip from the map, and convert it into driving directions before hand. Now ideally, if the maps are good enough, perhaps one could select some more direct routes then whoever gives those driving directions on the Internet. Then again, don't some of those GPS systems give some idea of traffic patterns? I thought I saw some advertizing some such features.
There's some info on that here
http://www.pinliang.com.sg/navigation.htm
The system also uses of Traffic Message Channel (TMC). The Traffic Messaqe Channel (TMC) is a standardised system of broadcastinq constantly updated traffic information (about accidents and hold-ups, for instance). TMC information is broadcast in many parts of Europe free of charqe. Sensors on motorways and roads measure traffic density and transfer this information to centralised traffic computers. Additional data such as direction and speed of traffic flow make it possible to predict conqestion.
Now they mention Europe, though I at least think I've seen it advertized on some systems in the US...