PhD culminates with a very different set of experiences and lessons for each person. Perhpas, it's too early for me to summarise my set of experiences and lessons, but certain thoughts do keep churning in my mind; and they seem to be conclusively true.
Research is like life. We all come into it with our own sets of strengths and failings. Before we can find out if they good for a research or not, comes the question of whether I know about them. Much tme and energy is wasted trying to tap sources of strength which are already dry. One may face severe lack of confidence and depression due to the above mistake.
The second learning is that it's not necessarily true that people who never make mistakes, never ever get to question their capability, are better off than others. Phd can't be evaluated merely by how quickly it gets over; how prompt and steady the flow of publications is. Just like life, PhD is to be judged much on the basis of how satisfied one feels at the end of it. Learning never stops. So it's wrong o say that people who don't do their PhDs don't learn. But, PhD is a duration when one is supposedly in the prime of his youth, is still open to lessons, and during the process, is pampered enough to keep believing in higher ideals of life. A PhD is potentially the example of a higher form of life, a germ of an evolved creature. The world is not evolved enough to want to be like a scholar; but perhaps is wise enough to protect and honour a race that's the only concrete hope that a higher, more evolved form of life is possible. Well! That may sound rather egotistical! But well, that's not meant to offend anybody. :)