I have a tom tom unit. In London I rarely, if ever, use it. Partly this is down to a broad familiarity with the places I am going, and partly it's because London is probably pretty well-signposted, and even if you do make one or two wrong turns, you can get to where you want to go pretty easily (if slowly).

Outside London, even just on a long motorway journey, it's can be a very useful addition to the information available in printed maps etc - personally I like to know exactly how far you have to go and how long it's going to take). And in line with the examples that Michael gives, abroad I have found it to be indispensable - particularly in rural France where the signposts are few (and sometimes contradictory).

The key thing is that these devices can provide valuable additional information, and if used properly, they make journeys faster and in some cases safer (having spoken directions is surely better than driving with an A-Z open on your knee?) and less stressful - for example, where you arrive in a town with which you are not familiar and you are trying to navigate through a convoluted one-way system.