As someone who got spanked quite a lot(I wasn't a brat...well, sometimes...heh) I would say that spanking being good or bad really depends on how it is used. I was spanked a lot, yes, but never for unfair reasons. Also, both before and after spanking, my parents explained why, let me know they still loved me, that they didn't hate me, etc... Sure, it hurt for a little while, but I was never permanently damaged. I think it's kind of funny that the butt is one of the few places where you can get hit hard and not hurt for a really long time...*speculation*
However, I would draw the line there. I think it's important to let a child know why they're being spanked, and they need to know that they're still loved as well. If a spanking is just given, to a child it doesn't really make sense. They were punished, and they may not know why, and it'll just become a negative memory. The whole point is not so much to hurt the child as to reinforce the memory. Pain teaches someone not to do a wrong thing twice. It's one of those survival things...and really, what punishment we get as kids is so much more lax than what we'd get as adults. I'm glad that I've grown up with spanking, because I learned from it. I'd really much rather have a little pain in my childhood then have learned that I can get away with things and end up in jail. It's so incredibly important to shape a child in a correct way, and a loving way. In all my life, even as a child, I never doubted that my parents had good reason for punishing me, though I did hate it.
The whole problem is that there are parents who don't use punishment(and therefore spanking) correctly. I'm not a parent, but even I can tell when spankings are used wrongly. Sure, it's up to the parent, and you'll never see me correcting a parent(I'm 16 for heaven's sake) but there's a fine line between abuse and positive reinforcement. It really comes down to whether a parent is dealing out punishment through anger or through love. It makes a world of difference...
I love this...in school, a lot of us juniors and seniors were debating spanking...and it was a really interesting conversation.