It's also called the hundred syllable mantra.
In Tibetan Buddhism, its one of the 4 preliminary practices used to purify oneself before going on to the next stage of enlightenment. It is said that after one chants the mantra 100,000 times, ones negative karma will be removed.
My experience with the mantra so far, is having a dream with light appearing from the clouds shining down on all sentient beings.
I was wondering about other's experiences as well.