Not underground, but we have used one to search for open jet mines. Though fun to watch, as a tool it proved to rest firmly in the fat lot of use category. Walking the ground, as with most things, is a far better option than watching something on a screen...
The only time we found anything with it was when we weren't even using it for that purpose! :bored:
As for underground, one of the main problems I can see is judging scale. We've used cameras on drain rods to probe sumps and it's extremely hard to guess how big the space you are looking at might be. Though that is in caves, in mines which are often have fairly consistently sized passages then that might not be such an issue. The thought of exploring remotely does fill me with a certain amount of misgiving though. Putting the armchair into exploring and all that... ::)