I actually studied with a former canine officer, who ran a local training group based on the book "The Koehler Method of Guard Dog Training" (I think that's the correct title, if rusty memory serves). I also raised one of his dobermans up until she was two years of age, then the dog went to his daughter. This was 30 years ago. He has since passed away. His wife still does local training using the basic Koehler dog obedience book, which is very much Koehler "lite" in comparisson to the guard dog portion of what Koehler taught.
Now, I've always been an independant thinker and will not blindly follow what someone else teaches. Most of my work with this doberman was done on my own and away from the instructor. I never got into the agitation or bite-work, only basic obedience. When he told me to "hang" (all fours off the ground) this dog for a minor infraction, huh, no way. He was generally a know-it-all-my-way-or-the-highway kind of guy in his early years. When I ran into him before he died, I jokingly said "Hey G___, hung any dogs lately?" to which he laughed and replied "I haven't hung a dog in fifteen years!". Well, I guess he was capable of growth after all.
Anyway, Koehler does have some good stuff if you stick to the basic obedience portion of his philosophies. He did use praise as positive reinforcement, but no treats. I still use the word "out", the line work, and especially his manuevers and turns when teaching "heel". He had his own basic form of a NILF type of philosophy, which I also still use.
Now, I do have some major issues with Koehler's guard dog book and some of the aversion methods. The darkside did include taping an object in a dog's mouth for chewing, dunking a dog's head into a water-filled hole for digging, hitting a dog with a leather strap as a punishment, and hanging with all fours off the ground.
Locally, I've seen better
results in dealing with aggression and leash work at the owner level from the dogs who have gone to my former instructor's wife's class than I have from the folks who are using the "positive" (not balanced trainers who are primarily positive, I'm talking about the extremist "positive only" type) trainers.
I think there is always a danger in following anyone blindly. I pick and choose lots of great tidbits from a variety of different people, philosophies, and methods. I agree with much which has been said regarding this and admire the folks who don't rush to throw the baby out with the bathwater.
So, that's my take on Koehler.