How far it's true that a happy customer brings 💯 new customers and an angry customer takes 💯 away?

Word of mouth/referral is free advertising and extremely important for generating new leads. 

According to a Nielsen study - "Word-of-mouth recommendations from friends and family, often referred to as earned advertising, are the most influential, as 84 percent of global respondents across 58 countries to the Nielsen online survey said this source was the most trustworthy."

This stat alone solidifies the word of mouth use case.