I've never used one, but from what I've seen of friends/family it's a hit/miss situation. If the dog runs off at full speed they'll just run right through the fence (was a large dog who loved to run). They'll get a shock, but at that point they're already past the fence and now the fence is preventing them from coming back into the yard. On the flip side of that, if the dog doesn't just bolt, it usually works. Once the dog is trained, I've seen owners being able to remove the fences and the dogs still never left the area. However, if you go this route I would keep a close eye on the collar. I've seen some of these collars rub the dog's neck raw.