To reiterate what Curtis said, Yes, they do work.
It's really quite simple, the dog learns - collar on, no barking - end of story.
Most dogs only have to get a stimulus a couple times and they're perfectly happy to NOT get it again. They know what the deal is and they shut their mouthes. Simple as that.
As mentioned above, the contacts can cause irritation if the collar is left on 24 hours a day. They're not designed to be worn 24/7. The irritation, though, is NOT a burn. The stimulus on a collar is not enough to burn the dog by any stretch of the imagination.
My only disagreement with Curtis (if you could even call it a disagreement) is that I will no longer us Innotek collars. I have multiple collars of theirs that no longer work and the company has no interest in doing anything to rectify the situation. I use Dogtra and Dogtra only. I have 2 training collars, one bark collar, and will be getting invisible fencing this summer - all Dogtra. I'm happy with all that I own. One other really nice thing about the Dogtra collars is they are now making them with vibration/page built in. This means my dogs really never even get an electric stimulus, one quick vibration (like the vibration of a cell phone) and they are perfectly happy to do exactly what they are supposed to be doing, no questions asked. This also means they don't use the 'beep' warning tone, which I don't like at all b/c there are other things in household life that make high pitched beeps and it confuses the dogs - only the collars give them a vibration right on their necks - they know exactly what that means.