mr kkm uses a hand held hp, with the maps on it (its a bit tooo technological for me so thats why it's mr kkm who uses it). we took it with us on the pennine way, and only got it out 3 times, when we were a bit lost....it took about 5 minutes to find us, and we were not usually more than 100 or so yards off track. used it on a day walk, again, more as a back up than anything else.
In all honesty, it is more of a comfort blanket than to replace maps and books - its nice to know that if something should happen we could give our location with some accuracy. there's also the powering issues. we have a solar charger, but i dont thing it was that successful.
hope this helps :-)