I got the citronella collar for Edie to wear at the office. Dogs don't like the smell, but the lemony essence is actually quite pleasant to people. It did stop her from barking, but the thing stopped working for some reason after a few weeks. They are expensive--the lowest I have seen them advertised is $99 and you buy the citronella refills separately.
You refill the citronella chamber from a pressurized can. Some dogs learn while their owners are away that they can bark for a while, empty the chamber, and then have a full day of citronella-free barking!!
Therefore, some people recommend that you use them only under supervision. My Edie never figured this out--
I was the person who recommended the Barker Breaker on another thread. It worked great for Edie. It is much less expensive, and just as effective if used correctly. You have to use it as soon as the dog barks so that he associates the sound with his bark. I have only one dog, so it works fine. I don't know how you would use it if you had multiple dogs. I'm not sure you would want to use the correction tone with the non-barkers nearby, but it wouldn't hurt them.
I don't believe in shocking little dogs, so I never tried any of those contraptions. I think if you can turn down the intensity of the shock VERY low, it would be just like a wierd tickling sensation on a big dog, but I'm not going to chance it on Edie.