What Do You Think? Y Combinator vs. Grad School
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If you like to go deep into problems, conducting basic science, and attempting to solve fundamentally difficult problems that may ultimately be intractable, and if deadlines stress you out and you feel like you don't get a chance to do anything "right" and if you have the ability to focus on the same thing for months or years at time, then grad school will probably suit you well.
On the other hand, if you more interested in getting tangible results fast, and getting other people interested in what your doing, and providing a product that regular people find valuable, and you're willing to change direction a lot and throw out good work for no other reason than that market isn't biting, then doing a startup will probably suit you well.
What keeps most people from becoming wealthy is not a lack of grad-school-type knowledge.
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.