From Lama Zopa Rinpoche advice:
The benefits of reciting the Compassion Buddha mantra are infinite, like the limitless sky. Even if you don’t have much intellectual understanding of Dharma, even if the only thing you know is OM MANI PADME HUM, still the happiest life is one lived with an attitude free of the eight worldly concerns. If you live your life with the pure attitude free of attachment clinging to this life and simply spend your life chanting OM MANI PADME HUM—this six-syllable mantra that is the essence of all Dharma—that’s the purest Dharma.
When a person who has recited ten malas of OM MANI PADME HUM a day goes into a river or an ocean, the water that touches the person’s body gets blessed, and this blessed water then purifies all the billions and billions of sentient beings in the water. So it’s unbelievably beneficial; this person saves the animals in that water from the most unbelievable suffering of the lower realms.
Read further advice on the benefits of chanting OM MANI PADME HUM.