I have five children. Each of them is different, of course. Each has a unique personality combination and therefore, they each respond differently to different forms of discipline and punishment. It took me a very short amount of time with each of them to determine what worked and what didn't when it came to behavior modification.
For one of my kids, just the THREAT of a spanking worked wonders. I honestly cannot remember EVER spanking her. For my oldest, no amount of spanking in the world would have made the slightest impact on her. I learned pretty quickly to find other ways of discipline, because to me - the END RESULT is what matters, not the mode of discipline itself.
My oldest son responded well to the few spankings he got.
I think the way the parent administers any discipline,including spankings, makes a big difference in the effectiveness. I am a pretty calm person about discipline. I NEVER beat my kids, or hit them in anger. I'm just not wired together that way - I never have used physical force to accomplish a goal.
Overall, my experience with spankings (which is pretty limited - I got a few as a kid, and they didn't seem to warp me in any way, and I have spanked my own kids a few times and they seem none the worse for wear) is that it's usually not the most effective form of discipline - BUT - every once in awhile, it's the perfect form. I reserved spankings for younger children and for outright rebellion.
For other things - I think it's more effective to make the punishment fit the crime. I like what I call CREATIVE DISCIPLINE (I can just see my children shivering at that term!). For instance, if one of my kids did something rotten to a neighbor kid, like call them names, I would make them go apologize in person. If one of them smarted off at me, I would make them write "I will talk respectfully to my mom" two hundred times. If one of them was prejudiced or judgmental about someone - say, someone who was obese, I would have them write a short story about how it might feel to be an outsider. If they didn't clean their room like they should, I would make them clean it and then do extra chores.
In other words, my goal was to increase productivity and awareness, and to make the child take responsibility for their actions. That's the difference between discipline and punishment. Punishment has it's place if the transgression is severe enough (and I did ground kids on occasion, or take away priviledges), but my ultimate goal was to teach my kids to think through why something was wrong, and hopefully curtail or stop that behavior in the future.
Spanking doesn't accomplish that so in my opinion it's uses are very limited.