Your most unhappy customers are your greatest source of learning. Is it true?

Yes, dealing with unhappy customers will be a great challenge to you and pushes you beyond your comfort zone. It also allows you to think outside of the box and it will be a remarkable experience in your career every time you get over an unhappy customer. I cannot really say the greatest source for learning but every unhappy customer that you deal with is always a new experience to you and it is more than any of your typical day with happy customers, so in my conclusion, you learn more with unsatisfied, angry customers rather than sticking to the usual and common talks with moderate customers.

TRUE100%. It helps to improve the customer care very efficiently and effectively.

yes ,

if you make him convert to a happy one

he will make any thing you want