The best thing that happened to women in modern times. I could probably even say that it's the best thing that has happened to mankind.
It doesn't leak at night as pads do, it's cheap and environmentally friendly, but mainly that it doesn't dry out my mucus membranes and doesn't contain bleach and pesticides that I don't want anywhere near my uterus.
A small revolution is what the menstrual cup is. No purchases, no smell. Freedom to just be for a whole day, no waste, always available, no chafing, no itching. Simply the best!