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.
Best protection for one who works in white clothes all day, never had a single problem and soon my whole department will buy one (I work at a hospital) each!
I have only used it during one period so far and I'm sold! Don't need to think about it, so nice to go and do what one wants and to not constantly have to think about if you have tampons with you etc... I love my cup!