I agree with the other reviewers that getting the stone/kernel out of a mango is the hardest task of eating a mango. But living in Brazil, I eat a lot of mangos and only rarely now use this device.
It's one thing removing the stone, but then you're left with mango flesh attached to the skin! It is fairly fiddly and messy negotiating that instead of the stone. I've found that the least messy way is to cut the flesh into little squares and then turn the skin inside out, making an outward curve of flesh pieces. But then you have to cut them away from the skin which is still not easy.
All in all, I find it easier to peel the mango and then cut it vertically as close to the stone as possible. But having said that, this Oxo device does what it should. Two tiny preliminary cuts from a knife at the top of the mango, and then push downwards, and out comes your core: it couldn't be easier.
But if you want to save on mess, then I think the knife and hands work better.