Only do a PhD if

1. You enjoy doing the PhD. This includes enjoying the subject, and having a very good adviser.
2. You see that you won't get stuck; that you'll finish relatively fast.
3. You have a full fellowship. No using loans or savings, at all. Nor should you fund tuition from work income, though it's OK to work as a teaching assistant and it's OK to make a little spending money on the side in a minor part-time job.
4. You have other career options available.

If all these are true, go for it! You can live the student life and have fun becoming the best in the world in something.

Background: I did a PhD in Harvard and am now working in something else. All four conditions are true for me. As far as I can, I would have done no worse or better in this career if I had gone straight into it from my BA.