They were the hot new thing 20-25 years ago, then kind of faded away. I made one and found it useful in an old, used Rem 700 that had a sewer pipe, pitted barrel. You could see the copper just looking down the barrel and as far as I could tell it took all the copper out and left all the pit bottoms very black. It didn't take too many shots to re-copper it though.
For general use, in my better rifles, I found it more trouble than it was worth. Plus I was always worried that the electrode rod in the barrel might somehow end up in contact with the barrel surface overnight, possibly damaging the barrel.