We have a Black Lab and a Newfie. We've had an invisible fence for over 7 years and it works great for them. My wife and I both work and the dogs are outside on our property on their own for 10-15 hours at a stretch.
The Lab is always testing the fence and she'll find out if the battery is dead within a day. The Newfie could care less and I doubt that he would cross the line even if we left his collar off for half a year.
It is so effective that you can throw a stick across the line and the Lab (who lives for bringing back sticks /forums/images/graemlins/grin.gif ) will stop dead in her tracks and give you a hurt look that says "meanie -- you KNOW I can't go there."
We had ours installed by the Invisible Fence franchisee who sold it to us, but it isn't much of a job if you rent a "ditchwitch" even if your soil is somewhat rocky, as ours is. The wire only goes a couple of inches down.
As to dogs learning to "beat it" on our fence, you can adjust the range of the warning (and shock) so that even if they build up a head of steam and try to run across it, they're going to be uncomfortable for longer than they want to. The Lab went through it a few times when she was young, but soon quit and we haven't had a dog out in 5-6 years.
True, it won't keep kids or other animals out, but given the size of our dogs, kids who don't already know them don't come on the property and they have been able to take care of themselves with respect to animals so far. Rumour has it that there are some bears appearing in Eastern Mass. and I'm not sure how they would do against bears. They're fine with anything else. If we had small dogs, it would be different.