We have a K9 system we have used for years, with few problems. Our dog is a small dumb shi-tzu, and does not challenge the fence at all. Once he had a couple of jolts as a puppy, he has never wanted to repeat the experience. The only problem now is that he isn't wearing the collar (because he always respected the invisible fence, and was too dumb to know when he wasn't wearing the collar), and he has 'accidentally' discovered that he is free to run around.
But once we put the collar back on him with new batteries, he only has to hear the beep once and he's well behaved again.
Are they humane? Yes, as I said the dog only has to feel this once or twice, very briefly, and that's enough. The collar has long probes (to administer the shock) which you use in the training phase, and shorter probes that won't actually shock him once he's trained. Having said that, I felt the shock once, and it was enough not to want to feel it again - not fun at all.
Hope this helps.
