i'm not against this method at all. by no means is it cruel or unusual. of course, it depends on the responsibility of the owner and their knowledge of using the device. we've used this method for our boxer just like an electric fence. at our summer house, the yard is unfenced and she likes to go "hunting" all the time. when she approaches the property line, our collars have a tone option that we press. then, if she continues, we push the zap button (yelling 'come' before each action).
i think a better analogy for a shock would be like shuffling your socks across the carpet and then touching a door knob. not painful, but it's annoying and makes you jump.
anyway, this has been a great method for training our dog. whenever she does something wrong, all she needs to do is hear the tone, and she knows that if she continues her wrongdoing, the zap will follow.
like you said, though, she's learned a thing or two over the years, and she knows when the collar is and isn't on