I considered getting one but decided against on the basis that so many others have had them for years often top of the range ones and most known good sites have been done to death.
On the brighter side it is good exercise and you may find it interesting if you come home with more that a hand full of tin foil, plus there is the "you never know factor". Give it a try if you wish but not for me as unless I find a pot of gold first day I will get bored with it in no time.
Considering how much of any value is located compared with how many detectors have been sold. I suspect you'd get much the same effect from walking around the countryside with a plate glued to a broom-handle saying beep at regular intervals only with a much lower initial outlay.
When I worked with BT I sometimes used a cable detector on copper/aluminium wires and also for old lead-sheathed cables when out in the field 'fault-locating'. Accuraccy was vital as it usually meant digging with picks & shovels or crowbars and cango-hammers.