I suppose it really depends what landscape you are investigating and the things you are trying to find. I find them useful for getting you to the "rough" area of the place you are trying to find, but after that its down to your own skills.
Of course if the rocks are covered over no matter how good your gps is you won't find anything, it still takes skill ,after that good maps, maybe previous references, etc... but part of the fun is trying to find something by yourself.

also geocaching..complete waste of time. what is the point of hiding a "lunch box" that is full of crap somewhere in the countryside, more often that not in a place of historic value?...surely putting boxes in those places means possible damage to the area, loads of geonuts running about , pulling rocks up, moving boulders, trying to find the sacred box of goodies, only to jump up and down shouting i have found it, i have no problem with geocaching, its good to have people getting fresh air, walking etc...but why insist on a hidden box full of crap...some geocaches are virtual, why not make them all virtual and save possible damage to historic sites or to nature.. i have seen some of these boxes and where they have been put...not clever.