I have never really heard of them working well. Seems to me, the dogs that are trainable to respect the fence are not usually the dogs that would leave the yard anyway.

The thing I hate about them is that ANY dog, cat, skunk, etc. can still enter your yard. I would NEVER let my dogs out in an open yard, even if they didn't want to get shocked by leaving the yard, knowing that ANYthing could come into the yard at will. Far, far, far too risky in my opinion.