I just ran across this old Chronicle of Higher Education piece, which offers some things to think about when trying to decide whether or not to complete your Ph.D. once you've decided to not pursue a faculty position.
I appreciate that the article relates the experiences of a few different people who left academia - some who chose to finish, and some who didn't. It also runs through a few important things you should take under consideration when you're deciding whether or not you want to finish your Ph.D. or not. Personally, if you only have six months or less until completion, I would finish. Similarly, if doing so would earn you a promotion or significant pay increase at work? I'd say do it.
But if you still have a long time until completion, or if your project is making you miserable, or if you simply don't want to finish? Maybe the best thing for you to do is just stop.
My close friend, S, is a grad student in a different department. She has been incredibly supportive of my decision to leave; however, she cannot understand how I'm content just dropping my dissertation and not finishing unless it's needed for a future job. At first, she kept trying to talk me into staying/finishing, telling me that I'd always regret it if I didn't. The funny thing, though, was that I didn't feel that way. I just wanted to be done and to move onto something else, and never look back unless I had to. That attitude was completely bizarre to her.
It took awhile for us to figure out why we felt so differently. Eventually, we figured it out. For S, earning the Ph.D. in itself was a goal that she felt compelled to achieve. Regardless of what she does with her degree after graduation (she's not entirely sure she wants an academic position), she knows that she wants that Ph.D. behind her name.
I don't care about that. I came to grad school because I wanted to be a professor ... and to be a professor, you need a Ph.D. Once I decided that I no longer wanted to be a professor, I suddenly realized that I didn't care one bit about finishing the degree. Getting a Ph.D. for the Ph.D.'s sake was never my goal.
Both attitudes are fine to have ... but it's important to realize that not everyone will feel the same way about quitting. S would never consider it, while I can hardly envision a circumstance under which I'd go back and finish.
If you're currently a grad student who is thinking about leaving, I highly recommend that you take a look at this piece, and think about how the completion of your Ph.D. fits in with your career goals and with what you want out of your life. Just because you started it doesn't mean you have to finish it ... but at the same time, if not finishing is going to make you miserable, then quit reading this blog and go work on it!!
Kidding, I'm kidding. But do go read the piece.