Gallup recommends a 3 question version. Net promoter, how likely are you to keep using services in the future and overall happiness. I like that balance. Happy doesn't matter unless you will keep buying. Keeping buying doesn't matter if the reason you are only one providing, but customer actually not happy, etc.
That does sound like a nice balance, but there's also value in having a single number. We noticed this going to our current two question system.
For example, with only one question, you can get anyone to rate at any time with a single click of a link (e.g. in an email). As soon as you have more than one question, customers have to visit a form with multiple fields and click a submit button.
This may not sound like a big deal, but it is. Also, having a single, easy to understand number helps us engrain the NPS in everything we do. Everyone on our team knows our NPS. Our experts even have their own NPS. They can recite each others' most recent scores. It's not easy to get an obscure metric like NPS engrained into everything like that.
That said, I do particularly like the question about repeat custom. We can measure it from our data, but intent to repeat sounds interesting.
