Knowing people from big families who were smacked (families of 5, families of 6 etc.) I think whether you do or don't smack your kids makes little effect on their personality. Difficult or mischievous kids stay that way and you may get momentary good behaviour or crying etc. which will stop them doing something in the short term but more than likely they'll be at it or something similar again quite soon.
I don't think smacking or not smacking makes a difference unless it's on abusive levels to how well behaved kids are. We are three siblings and never got hit. We were all scared of doing something wrong to upset my mum but in our very natures some were more troublesome than others.
I would never ever lay a finger on my child and I know there will be times I may want to but I'm trusting that because it's not learned behaviour (as in most people learn how to parent form their parents to a great degree) that i won't resort to it.
Either a child is too young to reason with or they can reason and either way smacking isn't right to me. It just doesn't fit into the equation unless there is some imminent danger and they won't listen.
However I don't think smacking is the sign of a bad parent it's just how some people parent.
But I am VERY impressed by those who were smacked as children who then go on NOT to hit.
Oh and I feel VERY awkward if someone smacks their child because of something they've done to my child (snatch, bite, etc.) I'd REALLY rather they didn't.
I do think what amounts to abuse though is a little difficult to determine. You could shout and scare a sensitive child into submission with not hitting and that child could go on to have a badly stunted emotional life so I think to a degree everyone should learn to understand that a child will do age appropriate things and that smacking them into submission isn't really what discipline is about.
I know a full grown adult who is the most horribly repressed individual and apparently he was a very joyous and normal (albeit mischievous and curious) child but the parents came down hard and he is now THE oddest human being I have ever met. He has no social skills, pretty much no friends and he's never really been able to hold down a job for very long either. He does seem like there's a lovely guy in there somewhere.
People who focus too much on good behaviour and discipline should always keep things in perspective.