IMO smacking is completely ineffective and potentially destructive. Says more about the parent than the child's behaviour.

DP and I have always been on the same page wrt no smacking. I can't imagine how you are teaching a small child (or anyone) anything positive by using physical violence. If my DD 'smacked' me she'd be spending some time alone to calm down! How is it okay for me to do it to her? I think it's an even stronger message that an adult is not allowed to use their physical strength and size against a small child than two children with each other!

I am not perfect and I have smacked DD out of reflex once. My goodness, did I apologise wholeheartedly after that one.