Is a PhD. Worth Six Years of Study?

This is the question Grew Mankiw answers in his blog, being a a professor of economics at Harvard University. Originally he asks himself:

Would you recommend someone to do a PhD if he knows he's unlikely to become a star in the field (weak math background due to lack of trainings and not being particularly gifted; lack of confidence in his creativity and talents) and does not have a burning desire to do research, but has interests in social science, enjoys learning, and likes to be able to interact with people he admires and respects? Or do you think it's better for him to work first until he's certain that research is what he wants to do?

And then he concludes that a PhD takes quite a bit more time and concerted effort than most graduate degrees and the passion for the study of any carrer you've chosen.