I did a PhD for the fun of it. And the challenges were so special and different and the whole crazy un-inuitiveness of academia (as opposed to the corporate world I was used to) was a complete breath of fresh air. I completed my PhD and went on to become an academic and never looked back. Its not for everyone, but if you like uncertainty, left-field and often unpredictable ways of thinking combined with talking with smart people all the time then it is a great place.