A good modern metal detector is light and well balanced. I never had any neck or back problems from them. You can walk in a normal position (just at a slower than normal pace). You don't really need to look down until you find something. All the input you need is coming to you through the headphones...not the dials on the machine. However, I think a lot of people do tend to look down as a habit searching for stuff sitting right on the surface and to make sure they are getting good coverage with their sweeps.
Speaking of sweeps, that is the one thing that will limit how long you can use the detector more than anything. To use one to its full effect you must constantly swing your arm back and forth. I would say its a good work out, but that is only if you are aiming to get one arm bigger than the other or you are ambidextrous:) I'd say an hour of use is pretty easy. Somewhere after that maybe 2 hours you will start to feel the effects in your arm.
Beaches are by far the best for really enjoy a metal detector. You don't need much of a shovel if any and if the sand is lose enough you can actually kick items up with your feet to avoid bending over. On the other hand, if you go to a grassy park after a dry spell and it starts to feel a lot like work pretty quick.
Also someone said you would only find bottle caps... With a modern detector and not much practice, you will dig up zero bottle caps. There is some trash that you will dig up, but the metal detectors are getting smarter and they get fooled less and less of the time.
Furthermore, metal detector users get a bad rap because of a few bad users. Not all of go around digging up historical sites or leave giant holes in the middle of parks. In fact I would say most of us leave places better than when we found them because we take some trash with us...if I find something trash or no I never throw it back.
As for returning stuff, there are lots of stories of graduation rings etc being returned to there owners after years and years. If it can be returned (it most cases it can't), a lot of people will try to return it. As I said, the hobby it is not really about money for most people.
Personally I have helped a person find a wedding ring (they knew where they had dropped within a 20 foot area) and another person find there car keys.
AmbergrisCaye.com 
