Its a sensitive subject, and unfortunately people tend to treat it dichotomously. There are two dichotomous statements that sum up the errors that are made on this subject.
1. Smacking a child = future bad outcomes
2. Disciplining a child = giving it a smack
For the first one, in studying large population samples scientists have found beyond a doubt that children that are smacked are more likely to suffer various negative outcomes than children who are not. It doesn't mean that a smack = disaster for the child, it means that a smack increases the risk that the child will experience any of a number of negative emotional and mental issues into their adulthood, and/or a variety of behavioural issues.
So it comes down to risk. You may well be a person for whom smacking didn't effect you negatively, and so might all your children. But the risk exists that one or more of your kids will retain negative affect from being smacked.
For the second statement, if a parent decides not to take that risk, but makes the mistake of confusing smacking for discipline, they will very likely raise a problem child who becomes a problem adult. Child require discipline, there is no doubt, the key is to understand that there are many methods of discipline available, and smacking is just one of them, often not the most effective.
I don't smack my daughter, never have, my wife has a couple of times. I have her respect and she is quick to obey me. The first time she had a major tantrum I carried her into her bedroom, sat her down, gave a box of tissues, and calmly packed almost all her toys into boxes. She calmed down pretty quick and asked me what I was doing. I explained that I was taking these toys away for a while, because he behaviour was wrong and she needed to be disciplined. I then put the boxes in the garage.
She got her toys back over the next few weeks, a bit at a time, and she hasn't done a tantrum since.
I not against smacking per se, I've just decided personally not to risk it. I prefer to get to know my child and understand what works with her. If there is one situation in which I would be 100% against smacking, it would be when it clearly doesn't work. If a parent is smacking their child for the same reason over and over, that is plain wrong. The definition of insanity, I think it was Einstein who said, is trying to fix a problem using the same failed solution over and over again.