im pretty good with maps,can identify any gradient, feature etc, but have found since i have sat nav, when going to unfamilair areas, it takes all the 'sh**, i should have turned there' out of it. You just need to use common sense and a bit of inteligence with them. I could go back to not having one, and if im going to an unfamiliar part of the country, i will always look at a map to get a general direction, but it's when you need to find 34a fred bloggs st in a town 200 miles from home, just takes the stress out of it. I have used mine for several ebay driving missions (going on one of these tomorrow after work to pick upa pair of tannoy dual concentric speakers, in Preston), and though i know where preston is, i dont know it as a town, and i know i will get to where i need to be OK.
I dont always have it set to a destination, for exaple last sun, i decided to have a run from stoke to devon, and didnt use it at all, apart from using it as a speedo 9more accurate than my punto speedo) and for warnings of traffic jams, speed cameras etc.
I do feel they do makea safer driving environment, no matter how we are not supposed to, i bet most people have been reading a map or written directions for detail, but before i head off somewhere, i do tend to use a map or existing knowledge to plan which mway or A roads i need to use, and use sat nav for the 'strange town navigation',and be aware that they do occasionally try and send you the wrong way down one way streets, sat navs are very useful, but use with inelligence.