Jilly,
I have used one, but not for lost coins. I borrowed my neighbors metal detector when an old building on our property came down. I needed to hunt and find all the nails so I didn't get a punctured tire in my lawn mower.
It worked better for my neighbor than me. He came to help me and found an old small piece of tin about 6 inches underground. It did find some of the nails. Of course, it was an older model and hadn't been used for a while. I don't think I had it set correctly. It was fun. I had hoped to find old coins, but I guess none of my relatives, or former property owners, had hidden any.
Have you tried using one?