Generally I don't smack as I think its hypocritical to hit a child but tell them they are not allowed to hit others. I also don't want my kids to comply out of fear, but out of a moral/social obligation to behave appropriately and treat others and themselves with respect and compassion. There are far more effective ways of disciplining I think.
I have smacked though, and no doubt will again. I am slowly convincing my husband my way is best, he was belted as a child and 'turned out ok' so always expected he would use this form of discipline. I think he is finally seeing how much it harmed him (psychologically) and that there ARE other ways of doing it. Yay!!!
I get a bit of a hard time from friends/family for not smacking, pretty much everyone we know smacks sometimes (no-one smacks a lot) and people have said when my kids have gone through 'difficult' periods its because we don't smack them. But I see the fear in their kids' eyes when they smack their kids and I don't want my kids to be scared of me, or scared into submission.
I don't really subscribe to the 'if you hit your partner it'd be domestic violence' train of thought. Kids and spouses are not the same. I don't believe kids should be treated the same as adults (I would never tell my husband he had to go to bed now because he is tired and cranky and has work in the morning, or withold money from him), but I do believe kids have a right to feel safe. I also believe (as per other thread) that yelling in the place of smacking is just as bad (not yelling to a child from another room to come and do something or to stop a child hurting themselves).