I personally have a metal detector and use it quite often.
It is true, sometimes you search for hours and cant even find a modern coin, but the thrill of finding something and realising it is something old or valuable (jewellery, silver coins) really makes you think that its worth it.
I myself didn't find anything incredible yet, my best find was a Prussian 2 1/2 Silbergroschen coin from 1843.
Not particularly great, i know, but the fact that this coin was out there for over 150 years and no one found it to that point is enough to keep me going
Its true, its not the most rewarding hobby, but everyone that is interested in it should at least try it once, maybe its something for them