Instruct: Classify a given Amazon review into positive or negative sentiment.
Query: very effective, but might not work for very challenging dogs

This collar worked great for my smart and trainable long-haired German Shepherd -- effectively re-trained her barking behavior, and we quickly got to the point where she didn't need it any more. I also loaned it to a friend with a very spirited Golden Retriever, and it has worked well for their dog, although she continues to need to wear it most of the time even after a year. My mother tried it with her keeshonds, who are the most stubborn dogs I've ever met, and it really didn't work with them, even when she made adjustments for their thick fur. Of course, those dogs are pretty behaviorally challenging, and a strong shock-collar has only been moderately effective for them. So ... for a spirited but reasonably compliant dog, it has seemed to work well, but for more stubborn dogs, the barking challenge may go on and on.