Training with electric shock collars

I know several people who use these things to help train their dogs (in particular if the dog is not kept on a leash outside and tries to run into the road, or over the neighbour's fence for example.

I like to hear if any dog owners have ever found a need to use such devices, and if they were effective at all.
Personally, if I had a dog, I'd probably be more inclined to smack it on the nose with a rolled up paper or something. Seems a lot more humane. Then again, who knows? I'm sure the collars have to be unpleasant, otherwise the dog would just ignore them!

Comments?