Did you learn anything in your studies at that level that you still use?
I'd have to say 'no'. I did a DPhil by pure research and had to produce a thesis of original work. That means that I did my own learning with some guidance. My thesis is all about provably secure computer systems (using CSP) and I developed an algebra for reasoning about secure systems.
I've never used any of this stuff.
There are plenty of things that I learnt as an undergraduate that I use, however.
That's one way to look at it, the other is to understand the analytic thinking, rigor, patience, new ideas you get out of it -- these are hard to quantify.
But I think I got most of those things from the undergraduate degree.
I think it's a question of sharpening research skills. It's a qualitative deepening of critical thinking, and the step far beyond passable or great to great and original.
I think it's a bit like comparing high school math with university math. Or like saying that you can program after taking a single programing course. :)
Similar skills can be gained elsewhere, are other skill sets are often needed in industry.
I have to agree with the rest - do a PhD if you clearly want to do research/work in academia. If you're just doing it for the credentials - stay far,far away. You might survive, but you will be utterly miserable.
But.
I don't think those three years were a good investment when seen from a start-up perspective. I think three years of extra real-world experience would have been more valuable.