I don't SLAP or HIT my children. But there has been the occasional smack. Anyone who panic smacks or slaps their kids without warning needs to take a look at themselves and their self control. When my children need a smack they are sent to their room to wait. They are then told why they are getting the smack and more often then not, express remorse and just get a telling off. A smack is the last course of action too - if there's another suitable punishment then we stick to that.
Every child is different too - my eldest has only ever had a couple of smacks in her life (she's seven). This is because her temperament responds to our tone of voice (that and she's really well behaved). My five year old son gets up to a little more mischief and has had the occasional smack. He's now at an age where he knows he's done wrong and responds. If his action warrants a smack then a 'pat' on the back of the hand is enough. My three year old daughter is feisty and stroppy. When she's having a 'moment' then a smack won't help and she's given time outs. When she does misbehave (such as pushing other children over) then she is firmly told off, then told she will get a smack for being naughty and only then smacked on the back of the hand.
Kids should never have to fear their parents. If you do smack your kids then you shouldn't be trying to inflict pain or shock them by catching them off guard. Everything you do at this early stage of their lives will effect their behaviour and how they deal with conflict with others later...
For the record, I was smacked and occasionally hit. I don't begrudge my parents doing it, they did their best with the parenting resources available to them a the time. I take my own approach with my kids based on what I have learnt myself. As I said, kids are different - some respond to verbal, others need physical discipline. It's how you apply this as to whether it's right or wrong.