Success with pay-what-you-want model?

What experience do you have with a "pay what you want" model (that is - when the user can choose how much to pay for your product) or revenue through "tipping services" like BuyMeACoffee?

• What has your approach been? Is it more of a "maybe not worth charging for this, but making it open for donations anyway - because who knows!?" - or a deliberate strategy?

• Was it completely voluntarily or did the user have to make a choice to e.g. access certain features or download something?

• Any examples of when it did generate more revenue than expected?

• Examples of when it completely failed?

Would love to hear!