I also have an invisible fence and we live in the country. Because our roads are not high traffic I feel it's relatively safe to rely on the fence to contain Moe and Tally. Both, however, HAVE gotten out of the yard on rare occasions. They only go out into the yard when someone is home so I have been almost immediately aware when they have gotten outside the fence. I can tell you I nearly have a stroke when I imagine what COULD happen to them. Our niehgbors all use invisible fence and we all know each others dogs - I had someone knock on my door to tell me about Moe overstepping his boundaries (no shock collar that day) and I have called my neighbors if I have seen thier dogs loose. We all look out for each other. And, like I said, there is little or no traffic on our road. It is also true that invisible fence is no protection from animals who could come into your yard - other dogs, skunks, raccoons, coyotes, etc. Another reason that Moe and Tally are never left outside if nobody is home...
As someone said, some dogs don't do well with an invisible fence. My sister-in-law's siberian husky learned early on that it only hurt for a second and to her it was worth the pain to roam free. It took a while but after ten years or so she was hit by a car. Also, should your dog get out, it may not be able to get back in. Sometimes a dog will run through very spontaneously (say, after a cat) and may not have the same motivation to cross back into the yard.
If I lived on a busier road I would NEVER trust my dog's life (or children's) to an invisible fence - I'd invest in a real fence. You'll need to evaluate your own situation before deciding if it's for you.