We have the Invisible Fence (that actual brand name one that they come and install) and love it. Elvis has never gotten out and actually seems scared of it, even though we only have it on the audible beep setting and no shock is delivered. He learned pretty quickly to run away from the beep, and to this day stays about 5 feet back from it. In fact, when we moved and had it set up in the new house, he remembered the orange flags and knew his new perimeter within one day.
However, Elvis is never, EVER outside without us. He never wants to be, but I would worry too much about him. Back in WV our neighborhood was full of little kids who loved Elvis, and I'd never forgive myself if they came to pet him and something happened - like Elvis innocently knocking one down or something. I'd worry about someone taking him. I'd worry about another dog coming in - here in our new neighborhood our next door neighbor has a dog who hates any other dog. Luckily we get along really well and make sure that we don't bring our dogs out at the same time.
All that being said, we do love it. We have him outside with us when we're doing yard work or relaxing on the porch, and when it's rainy/snowy he can go out, do his thing, and come back in without us going out in the cold or getting wet too. It also came with an indoor disk type thing that you can put anywhere in the house you want to keep dog free. We set the radius to 3 feet and keep it on my husband's desk since Elvis has a taste for term papers and water bills.
Since you seem to already have a fence and want a back up, this may work. However, if they're determined to jump the fence and run, it may not. Growing up our dog (a mutt) would break the barrier all the time and take himself for walks. When he returned he would sit on the sidewalk and wait for us to remove his collar so he could come back in the yard. He was determined enough to get out, but smart enough not to come back in!