Toby had aggression issues and so I took him to Obedience classes and the trainer recommended that I use to gentle leader to keep any accidental bitings with other dogs. It worked great but did mess up his muzzle hair when used to correct (Toby is in full coat). We used the gentle leader for Basic Obedience and for most of Obedience 1 class. But he still was being aggressive and growling with other dogs in the class.
We moved Toby to the pinch collar and he only needed on correction and he never growled or looked at another dog in class aggressively. So while the gentle leader will help with walk/pulling.....it does not correct aggression.
I can tell you that Toby is now in Obedience 2 and I am working him off leash the full hour among dogs like a Dobie, German Shepherd, Collie and some other large breed mixed dogs. He does not make any aggressive moves or growls toward other dogs in the class. The gentle leader helped with control and the pinch collar worked for the aggression.
Bottom line I recommend the gentle leader for walks.
JMHO