I had a golden retriever for a guide dog and she was one of the sweetest dogs I've ever known. Very intelligent, too. After she passed away, I missed her terribly.
One of my girlfriends had a husky for a guide dog. While she was excellent with her guide work, there were problems with aggression and excessive barking. Unfortunately, my girlfriend never did anything to stop these behaviors, so the barking and aggressive behavior continued to become worse. Her dog lived to be 15 years old and eventually had to be put down after an acute illness.