We stopped spanking when we had our second child. It was rare anyway, but there was something about her that we just knew that there was no way we could spank and have her turn out "fine". Certainly, no way we would ever have a good relationship with her.
It bothers me so much to hear people say "every kid needs to be spanked/whooped/corporal punished in order to be good". It's absolutely untrue and can be doing so much damage to so many kids.
Personally, I now think that it's always wrong. Do some kids turn out "fine"? Yes. But I don't think turning out fine is the measure of whether something is right or wrong. The entire purpose of why it works with some kids is that it makes them fear physical pain if they do something wrong. (I would much rather my kids do what I ask because they respect me and because they understand that's the right thing to do instead of just following the rules so they don't get hit.) I also have never in my life seen an adult spank a child who wasn't angry or annoyed. So, it's also teaching that if you're upset that someone has not done what you want them to that you can (and should) hit them.
I am a very logical person, so this was my conclusion once I looked at spanking objectively. There is absolutely no situation in which it would be appropriate for my husband or my parents to hit me in order to teach me a lesson or get me to obey a rule. So, to me, if it's wrong for an adult to ever hit another adult for those reasons, it simply doesn't make any sense why it would be appropriate to hit a child.