Is a PhD worth the effort? Is it worth the stress, the money and the time? Is it worth sacrificing potential career progression to pursue some obscure topic most of the world doesn't care about?
Unfortunately I can't give a straight yes-or-no answer. Personally, I think it was worth continuing when I wanted to quit, but had I quit and followed a different path I might have said it was the best decision I had ever made. I wouldn't have a PhD, but would my life be worse for it? I think things would have worked out OK.
In trying to answer whether your own PhD is "worth it", you've got to define what the "it" is.Is a PhD worth working hard for? Yes. Is it worth occasional late nights and a bit of stress? Yes. Is a PhD worth a career break? Yes, if you really want to do it and you can support yourself.
But is it worth genuine suffering?
In the last few years, I've spoken to some students in truly awful circumstances; whether that's because of crippling financial debt or criminally abusive supervisors. I've spoken to students who work every hour they can, wracked with guilt because they never see their kids, and students who suffer immeasurably through anxiety and depression.
Sometimes it is better to walk away; to write off the sunk cost and escape. If the PhD is not what you expected it to be, if you are constantly exhausted or making yourself ill, if your relationships are suffering, if you live in fear, if you feel humiliated by your supervisor, if you hate every day and if you are gaining nothing positive from the experience... I don't think getting a PhD is worth going through those things; especially when there's no guarantee of success if you keep going.
Research should be hard, but it should also be fun. It's a luxury to spend several years in pursuit of obscure new knowledge, playing with ideas and surrounded by wonderfully obsessive, interesting and often slightly weird people. If you never enjoy it, what's the point?
Ultimately, you have to decide for yourself where the limits are; what you are willing to sacrifice and what you are not.