I saw some videos of studio folk rubbing these on gongs and large drums to create horror movie atmosphere sounds. I don't have that at home. But I do have doors, windows, bathtub, kitchen floor, acoustic guitar, desk (didn't work on my dog). The sound these things make is mad, I love it. Don't tell your partner you have them and rub them on the outside of her bedroom door at 3 am.
I highly recommend them to everyone interested in sound effects and sound design. Experiment and have fun.