Disregarding the mental toll of a PhD (one problem being that too many people want to be paid to think than there are available positions), I totally disagree with the parent poster's claim that graduate school is a rip-off. Just being in an an academic environment, being able to take classes, & studying around really smart people and doing research at the highest academic level is a really great experience & makes the stress worth it.
Ok...
If you come out the other end of a PhD, then you have the opportunity for best of both worlds: high paying job to work on the problems you find intellectually stimulating.
Also, it is cliche, but doing a PhD does train a person in a way to think that you don't necessarily pick up in a normal job.
The thinking advantage of a PhD is really valuable, its like an extended apprenticeship with your advisor. Also, it gives you time to look at things you find interesting (well, it should, sometimes not).
I guess your mileage may vary. The best description I've heard about grad school is that it's a self induced neurosis. That was certainly my experience at the time I did it.