Yes, I had seen those type of alarms. However I don't think that they had a huge following as many people lost interest in false dog alarms. I think it would be better to have a real dog than a fake recording of dog's barking.
I have 2 real dogs, really small but they do the trick, I don't think many people are scared of them though.
I certainly have. I met a lovely couple on a campsite a few years ago, and they had a barking dog machine in their campervan which was activated if anyone got too close.They do seem to be a good idea.