I built one myself years ago when people started using them to hunt for gold. Later I bought one but all I ever found were bits of rusty old iron. I had more luck fossiking with a pan and later I built a sluice and also had a small dredge. Most of the areas I went to only had fine gold dust and flakes and the areas famous for nuggets were too far from where I lived. I still have some of the gold somewhere in a sample box and a jar. It was a hobby that got us out and camping in some nice places but cost far, far more than the value of gold found.
At about the time I was fooling around with metal detectors a guy in Victoria went out to the gold fields area with a few mates and a brand new metal detector. At a certain point they stopped at a roadside layby to stretch their legs and he announced that he would try it there as he had not used one before and had no idea how it worked. After a bit he got a big squeal and dug down expecting to find a bit of old car or rubbish and turned up a huge nugget that was worth a fortune. Talk about beginner's luck.