I ran across a great response to this question today_
Is it worth it for me to get a PhD if I don't intend to hold a job that would require one and my only reason for wanting one is so I know lots of stuff?
Some excerpts from the best response (a few comments down):
Absolutely not. I don't know what you plan to get a PhD in, but at least for my program you don't burn 7 years of your life doing extremely difficult and complicated work for long hours with little pay if you are not planning to use it.
In the end a PhD is only school for a year or two. After that it is a job. A difficult, time-consuming, high-pressure job with little pay and a time limit.
It is not a place to be unless you are absolutely, 100% sure it is what you need for the life you want to live. It is not something that a sane person does for fun, and certainly not what someone does for no reason.
I enjoy it, I really do. But I know it is what I need to live the life I want, the life I have wanted since before I can even remember. And I am sacrificing a huge amount to do it. I do not know a single PhD student who is not certain that this is what they need for the career they have chosen. Those that were not certain are no longer in the program. And there are plenty who have developed doubts since entering.
He's on a roll_
If your goal is to have fun it is even worse. You will be sacrificing a lot of potential pay you could be earning while working a normal job. When you get out you will have to compete with younger people who are more up-to-date on the latest material (since you would have spent the last several years focusing on an extremely narrow and probably esoteric topic to the exclusion of all else). It has serious costs that will hurt you unless you specifically plan your career path to take advantage of it.
These are the kinds of facts that everyone should know before they enter a serious PhD program. It's not extended college, it's not a lot of fun some days, and it certainly doesn't make financial sense.
That said, most days I wouldn't want to be doing anything else with my life.