I actually owned of these some time ago.
I was generally pleased, it worked well on a short distance and the dogs that I had to train reacted to it. On larger distances, it didn’t work so well. I can say that it will get the dogs attention, which is enough for a trainer, but if large vicious mongrel charges at you, I wouldn’t bet on it that it will stop the animal.
It is suited for training dogs and I would give it a much better rating if I didn’t recommend one to my colleague. In his case, the device didn’t work at all. Although I have a good experience with it, I can’t vouch that it will work 100%, especially on longer ranges.
My advice is that if you own a small dog that needs to be taught how to behave, this device is a good choice for you. It is not expensive and it won’t harm the dog.