It's only more efficient when I use it, which sounds like a joke about "it's only wrong when someone else does it." But if I am only buying a quart of ice cream and some toilet paper, I actually can scan those faster because I don't have to pass them to a person to scan them for me. My typical use case is roughly the same speed as a cashier. I also know how to use the machines fairly well.

However, your last sentence is exactly right. Once one idiot gets to self-checkout, the whole thing goes to shit for everyone. Or if something doesn't scan, or it is a bottle of Nyquil that some stores require ID to buy, then it is so much longer to get a person over to help you.