I've been sat-nav'ing for a few years..
The reason I started using Satnav was when I had to visit a lot of customers, often on sites in the middle of nowhere. I used to ues Autoroute, print out the overview map for general guidance, and a zoomed in just covering the area immediately around the desitination..
The problem was time and stress.. mainly roadworks/diversions, and continually scanning all road signs when in some built up areas, especially London.. Then if I hit a diversion, or a road block/accident, I was a bit thrown, I had to find somewhere to park, consult the map, etc, etc.. and when you can't afford to be late, it was just a PITA..
So I tried TOMTOM on my PDA.. and never looked back.. sure I can still use a map, but quite frankly it isn't a gimmick for some of us, navigating is just so much easier.. I know the general directions of course, but it's just so easy to have the guidance, and the times it's helped me circum-navigate accidents/congestion it's been a godsend....
To the poster above who nearly smashed his mates sat-nav up, not all sat-nav's are the same, and maps are updated every year.. I've met plenty of people with the same argument against sat-nav, but strangely, when they've been with me, even to places they already know the way to, they are very very impressed with it's accuracy and routing.. in fact, over 80% of our mobile staff now use TomTom and swear by it.. they keep a map in the car, just in case, but this only comes out once in a blue moon.. I've been sat in with a anti-satnav diehard, and everything is fine, providing they roughly know where they are going, as soon as the environment changes, and they are late, you can see them visibly stressing, checking out every road sign, and nearly having accidents with last minute lane changes..
You can of course get by without many time saving devices but I'd rather save the time for something more useful!!