I thought it was useful the few times I used it, because it did work to stop my toddlers trying to touch the stove. I watched them like a hawk, of course, and tried to keep them away, but inevitably they slipped my watch and got a chance, and while I managed to yank them away in time, I wanted to ensure they never tried again. So I smacked them on the hand while shouting at them. One never did try again, and the other tried once more several months later, then not again. I'm comfortable with a smack on the hand being better than horrible burns if they'd pulled a pot off the stove.
So I think it worked effectively as a deterrent to prevent serious harm. I don't think it's an effective 'punishment' for most kids, and other strategies usually work better.
Personally though, as a pp said, in my own childhood I much preferred a smack on the bum to being sent to my room for ten minutes or so – that deeply distressed me and left me screaming and tantruming, in a right state, whereas a smack was over and done with in a second. I think time outs or other consequences can actually affect children quite deeply. My cousin was smacked growing up, but much like me (she's also autistic), what she remembers as being very traumatic was being left to 'calm down' when she had a tantrum (probably a meltdown in hindsight) as an 8/9 year old, not being smacked. I agree with pp that many other things are much more harmful to a child than a smack.
Also, many parenting choices are all about the parent and not necessarily what's in the child's best interests.