There are many opinions about invisible fencing; in my own personal experience it has been positive. My dogs were very easy to train, and they are not being shocked. Bentley, my tri Cavalier, does not wear a collar because I fear he may have SM. He is trained to the flags and doesn't pass them even without the collar.
I certainly don't want to start an argument, but this is my personal experience. While there are dogs who have problems with repeated shocks because they are not properly trained to the flags and beeps, my dogs are not experiencing that. Please don't berate me on my choice to use an electronic fence, because I already know the opinions of most here. I only wanted to share my experience because it is different than what has been described here.