I use a cassette demag for my decks. I know that some folks say they may harm the heads; however, by the time that happens, I'll probably have a new one or some other piece of gear. Lots of folks still like tape, including me; however, the venue is slowly walking away down the avenue.
A friend of mine who had a high fi system was telling me how his tapes would not play very loud; I gave him the demag in a cassette and when he returned it shortly thereafter, he said he couldn't believe how much better his tapes sounded. He hadn't used a demag since he bought the system.