I have it, and we think it is awesome! We live on 1 acre, and our back yard backs up to a very busy road. We have a yellow lab. It's been great for her. She can go out to play whenever she wants.
It was expensive (I think around $1000 to put it in) but worth it IMO. As far a the shock...Maizie has been shocked probably 2 or 3 times total. The training is very systematic so that the dog learns quickly to stay away from the border. You start by putting up flags all around the border and walking the dog around the perimeter. Show the dog the flags and warn, no flags. Gradually, you remove the flags and you do have to let them be shocked one time to understand the feeling. Honestly, that was all it took for Maizie. If she gets close, she gets a warning beep that tells her to back up. She absolutely knows where the borders are, and doesn't go anywhere near them. Not for a toy, another animal or to greet a person.
No, they don't keep anything out of the yard, but that's not what they are for. They are made to keep your animals in. By the way, both DH and I have felt the shock. It's surprising, but not horrible.