A PhD can [and I insist CAN] make you very insightful and very diligent as well. It CAN also make you see things in refreshingly new angles.
If this is what you seek to become; then go for it.
If, however, you are looking for additional knowledge in you field of specialisation, a PhD will be a very disappointing experience. This is because it teaches you very little extra above a master's degree.
It is said that the higher you go in education, you learn more and more about less and less. By the time you complete the PhD, you might know everything there is to know about nothing!
Nothing is real unless it can be named; nothing has value unless it can be sold; money is worthless unless you spend it.