They say your mapping is only as good as your data. I experienced this fist-hand on my summer holidays this year. [Some of my Flickr! photos is posted on a Yahoo! personal map].
We headed for north Wales and the Wirral (near Liverpool) UK with a map book and a sat-nav, and that worked perfectly like so: set the destination of the day, the driver listens to the vocal directions, the passenger watches the dynamic map aganst the map book. We were thus able to adjust minor miscues, and mostly keep thinking for ourselves, augmenting our hunches with data. And being on holidays, we laughed off the few unwanted detours as mini adventures... Likely not the reaction of a truck driver or business person trying to get somewhere on time!
Visiting Lake Balaton and Budapest HU was different. We used someone else's sat-nav, and were warned that there was a new bridge and a new highway near Budapest. We were told not to worry, however, as that wouldn't help with our travel times anyway... With no map in hand, we were at the mercy of the sat-nav, and promptly ended up the wrong side the Danube! With no back-up map and with high speed traffic, we ended up with quite a few wrong turns. The scariest in the end was that we gave up our thinking, as we had no checks against sat-nav directions...
On the bright side, sat-nav has evolved a lot in the last decade - indeed become a consumer product, in other words a Mappliance (= map + appliance) I tweeted earlier - and rarely do we experience the pitfalls I collected a decade ago for a GIS Day talk: It's the data, stupid! indeed.