There isn't a clear cut answer because so much depends on your personality and what you want to get out of life.
What I can definitively say: if you've been out of school for a few years, getting an MS in CS is totally worth it. I learned so much about the "new stuff" that I never got around to when I was working, and the bigger toolbox has already paid off for me.
Beyond that, I sort of agree with other posters, but I'm a little more cynical about the long-term vs short-term aims of academia. I'm finding that my startup sensibilities work well for guiding my efforts in a PhD, but this is because I've come believe that grad school is essentially a machine for conferring reputation from professors to students. Keep in mind that even when your advisor gives you a lot of latitude (and that'll take years), you're still not your own boss... and that can suck if you're older than the average student.
Of course, working in a research lab has similar issues. You don't completely get to set your direction, there's a big bureaucracy, and you have to work hard to get people to understand your work.
Out of everything I've done, being in a startup where we all shared The Dream was the best.