They are really hard questions to answer, I can't say I've ever been in a position where i've physically hurt my kids. I don't smack but used to hold them firmly to restrain them as toddlers. Now that they're older I find other forms of punishment like taking something away for a certain amount of time or banning playstation or wii more effective. I never make threats I can't follow through with and the kids know that if I threaten them with somehting it's going to happen and usually stop the behaviour.

But everyone is different and I don't ever judge anyone who smacks their kids.