Only deters barking while it's on
933130
My vet recommended this type of collar to control barking for a dog I inherited. She is a small, 6 lb chihuahua who barks at everything (and seemingly absolutely nothing!!). The collar was simple to adjust to her size (although it barely goes small enough for her) and to juice up with the citronella. Her first couple times wearing it promptly quelled her barking. However, as soon as I take the unit off, she goes right back to her usual barking (constant!). After several months of trying this collar, it only "works" to stop barking when she is actually wearing it. It doesn't seem to be a training device, just a barking inhibitor when she's wearing it. Plus side:no barking when it's on! Great for night time or naps to get some rest in a quiet house.
2407818
The name is misleading - this product does not emit an "ultrasonic" sound.  Even set to the highest frequency setting my wife and I can hear the pulse.  I bought this to keep my dog from digging in the corner of my yard, placing this unit right in the corner.  It does not deter her, she just spends her time barking at it and is already starting to ignore it.  The flashing light is a waste of time, it strobes once or twice and then stops.  As I said earlier, the sound emitted isn't ultrasonic, just an annoying chirping sound that bothers me as much as it does her.  Would not recommend.
4251945
Doesn't work didn't even deter my dogs barking.
2428092
Just to be clear - as far as effectiveness goes, this is a five-star product.  My two dogs stop barking after one or two barks when we have this on and we've seen a dramatic decrease in barking in general even without the machine on.  (Small note - one of our dogs is a 6 lb. chihuahua and it seems as though the machine occasionally can't pick up his bark, but this is only about a quarter of the time).

However, our dogs are so terrified of it that I feel guilty using it! After it goes off one of our dogs will walk around with his tail between his legs whimpering for hours (not an exaggeration).  He's kind of a nervous guy in general so maybe this isn't a normal response, but it's enough to make me hesitant to turn it on unless absolutely necessary.
2022929
I had high hopes that this product would solve the noise problems from my dog, but what it did was stop the barking and create an even worse whimpering problem. The challenge is the collar does assist in curbing the barking tendancy, however it does NOT go off when the dog whimpers or howls. Our dog has figured this out and now whimpers and howls more than he ever barked. Although this was a reasonable cost for a potential solution, I recommend spending more for a collar with a remote where you have control over curbing bad behaviors.
627898
This version does not go off continually. Even with the audible volume turned to the lowest setting you can still hear when this thing clicks on or off. No matter what I do I cannot keep this "ON" continually. I'll set it outside when the neighbor's dog is doing his thing and it takes about 30-40 seconds to kick on. Then I'll see the dog twist his head and walk off.  20 minutes later he's back. I'll sneak up around the side of the house and the dog continues to bark at nothing. So sneaking around where the dog won't see me, I'll wave my hand across the front and I'll heat it does nothing, i'll move it and it may or may not kick on. If it does kick on the Dog stops barking and walks off. So no matter what this thing is tied to some kind of motion detection (even when turned to the always-on setting). Or this thing turns off and back on randomly. Overall I am noticing the dog spending less time on our side of the fence so it must be turning on "some" during the day. Should have bought the version specifically for dogs with the remote control but I fear it's the same shobby craftsmanship. And yes, there's a direct line of sight between the device and the dog next door, he's about 15 feet away from it.
And don't think you can be sneaky about this. There's a red LED that is on when in motion detection behind the cloudy/white plastic, and the LED is on all the time when set to the always-on mode. You can't cover this portion up as that's what the motion detection shoots through, and I also assume the day/night sensor.
2726588
This is a waste of money. Doesn't stop neighbors dogs from barking. Light comes on to show it's working but has zero effect on barking dogs.
2766491
I tried using this to get my border collie/Australian Shepherd mix (50ish lbs) to stop barking at other dogs and buses on walks but it didn't even bother him. The only reason I gave this even 3 stars is because it turns out it works great on my two cats who keep stealing food from the counter and waking me at 4 am. As soon as they see me reach for the egg they are off the counter or back on their tree when I'm trying to sleep. As for the dog, I have yet to find a product or method that stops him from barking like a crazy person.
4587737
It works for the bark control, however it seems to go off when she is not barking and just scratches herself or jumps down from the couch. I feel like it desensitizes her to the noise when it happens randomly. I even put it on the lowest sensitivity sometimes, but even then it will go off unwarranted.
1667940
This device did not deter my dog from barking. She is an Australian Cattle Dog and quickly figured the device out.  The first time or two the collar beeped, she stopped barking, but that was it.  By the time the collar got to the shock, she had already been barking for over a minute.  She quickly learned that she could bark for almost a minute before being shocked, so she would stop to let the collar reset, and then start the process over.
We have used the PetSafe anti bark collar in the past and that worked a lot better than this product, but the batteries do not last long and are expensive to replace, so that's why we tried this product.  It may work if your dog is easily scared by the beeping.
1973040
I bought this for my very territorial papillon.  She is obviously very bothered by the ultrasonic noise and immediately quits barking and hides whenever it goes off.  However, it rarely goes off on its own.  I usually have to press the manual control to get her to shut up.  Sometimes it'll automatically go off (and sometimes it goes off at nothing) but it doesn't seem to be able to differentiate between barking and other noises very well.  The good thing about it is that I can easily take it off of the wall where I mounted it and carry it around the house with me.  So, if I have company, I just need to keep it near me so I can set it off whenever she starts barking at this unwanted guest.
4480055
I bought three of these collars for my dogs to curb their barking. I have adjusted the collar strap, the sensitivity, the positioning on my dogs, and the prong length, all to no avail. The only time these collars even activate is when my dogs are itching their necks and it gives the warning beep that should precede the shocks. My dogs can be barking incessantly and the collar will not even activate the warning beep, much less the shock(s) that would be required to actually do something.
2222087
We got two of these and they work as advertise. The dogs will not bark when they have them on. One of them seems to have a low tolarance and goes off without the dog really barking. I now just put it on the dog and don't turn it on and it still works. The dog won't bark.
1910922
I bought this because it was 1/4th the price of anything else (got it from a chinese website).  Like the other reviewer, it is for my chihuahua that barks all night.

First off, the bark control has an ultrasonic mode and an audible mode.  You will want to have it on ultrasonic mode to control barking.  Get a quarter, turn the large plastic cap off of the back, and insert the included batteries.  Next, press the On/Off button.  A red led will light up.  Measure the collar and attach it to your dog.  Make sure it is in ultrasonic mode by pressing the ultrasonic button.  Your small dog should now have a sudden aversion to prolonged barking.

Pros:
-Cheap and it works
-Led light lets you know from a distance if it is working at night.

Cons:
-The included batteries died within 10 minutes.  I had to go to the store and spent over $5 on two lr1131 batteries.  Amazon sells a 20 pack for $6.  These are all the same battery and any two will work in this bark control device:  AG10 LR1130 LR1131 389A 389
-The battery cap was amazingly hard to unscrew initially,  I used a quarter and my large strong hands and still had difficulty unscrewing the cap.
-The unit is large for small dogs (as the other reviewer has said)
-The collar is elastic with plastic connectors.  After one day, the collar was loose on my dog.

I expect that the collar will eventually fail and I will probably just use electrical zip ties to tie it to my dogs regular collar.  So far, I have had it on my dog for 5 days and the (replacement) batteries have not worn out.
3833923
Catches a lot of background noise and my little dog welps and causes herself more tor ture than is necessary when it jolts her. She barely wears it now but fears the sight of it....doesn't keep her from barking though. Works like it says it will.
4673562
In spite of reading all the reviews for this, I decided to give it a try.  One of the most-favorited reviews said that this unit works best (or only works) for deep, loud barks, which is what our neighbors have.  We set it up, tested it, and let it go.  And...nothing happened.  The dogs barked just as much every night.  I'd go out in the freezing cold and see that it was being triggered by the barks, but it did nothing to stop them.  Moving it even closer to the dogs didn't help.  Sadly, it's worthless.  Also, it's not weather sealed, so eventually I assume the battery compartment and electronics would be ruined by moisture.
3710973
Updated review:
3 stars

There are dogs this machine can almost never detect. Small irritating yappy dogs almost never set this thing off even if they're very close to it. It's more likely to go off from the sound of far away airplanes and motorcycles than a small dog barking right in front of it.

To reduce the dog's barking a completely different strategy seems to help. Grow a very tall dense hedge so the dog has trouble seeing you and you don't have to see your neighbors. And put up a variety of different toned wind chimes near the offensive dog. The chimes will cover up your sounds and the dog won't know when to bark. A tall privacy hedge would make sense even if there were no barking dogs.

-------------------------------
Original Review:
5 stars

After leaving this thing sitting outside for a year, I switched it to audible mode (from ultrasonic) because it wasn't having any effect on the dog. I found the batteries were still strong and the sound to be sharp and annoying. The dog suddenly seemed to start noticing. Maybe it was ultrasonic deaf. Audible has the added benefits of letting you know it's still working and will punish your neighbors too. BTW I also have mine setup for maximum sensitivity and to activate after a single bark.

If it were $20 I'd aim 3 or 4 at each fence that has a dog. If you have a lot of money you should consider this. Right now I only have 1 aimed at the most savage dog. The build of the product seems reasonable. Its weather sealed so mist and humidity don't rust out the electronics and other parts. I don't know how it would do completely exposed to the elements because I keep mine a little off the ground and covered from rain; the blow horn is not obstructed by the cover.