I did download a detecting app at one point, but haven't used it yet.
I use a Garmin eTrex to mark the odd way-point. I went with a "real" GPS for safety reasons, I didn't want to rely on cellular service for functionality.
(I realize most phones have some sort of "GPS", but there is concerns they will not function right without cell service or adequate data available.)
That being said, a cell phone is probably nicer to use, I will watch this thread to see what others are using.