Although it was a little while ago now and I'm sure things have improved, I did a little 'test' to see how accurate my recently purchased GPS was. I was alone, in the Lake District in January. The visibility was occasional and fleeting - only when the snow and thick cloud lifted for a moment. Using a map and compass, I walked up to the top of Great Gable. I knew exactly where I was - right next to the Fell and Rock Climbing Club War Memorial, a feature marked on the map - and I had a look at the GPS. It put me at a point some 50 metres away which, on the ground, was over a vertical drop. Needless to say, I was not impressed: if I had relied on the GPS that day, I could have ended up in all sorts of trouble; thankfully, the tried-and-tested equipment, along with a little navigating skill, got me to the top and down again safely.
It is when one is 'lost' and therefore under a certain amount of duress that one relies on equipment to help you out and clarify where you are. Although by no means 'lost', if I had relied on the GPS or hadn't had a map, its inaccuracy would have added to my confusion and worsened the situation. I have not used the device since.
Although more modern devices are, I'm sure, more accurate and reliable, they are also easy and fun to use. This increases reliance on them and pushes the trad. nav. skills into the background. But the devices, however good, rely on battery power and, in some examples, are not too great when the weather is wholly against you - perhaps the very time when one most needs them. If they fail you then, your safety can become dependent on luck, rather than judgement.
By all means carry a device but make very sure that you also carry the relevant map and compass and, perhaps most importantly, know how to use those last two confidently and accurately. they could just save your life.