I wish my neighbour would buy one of those collars to train her mutt not to bark. The bloke a few doors away could do with one for his dog too... and whoever it is who lives at the back of my house that lets their barking dog out into the back yard at 5am every day.
Craft, that's because in order for it to 'work' you need to use the exact timing skills that would be used to train 'normally', ie without it.
An owner needs to be tuned in to a dog's behaviour and know what makes it tick, what stimulates the nuisance barking? If they have worked that out then they should be able to stop it without resorting to one of these collars; if they haven't worked it out and try to use a collar it will just confuse the dog.