Here's my experience: I started a Ph.D program in computer science (focus in systems and/or programming languages and/or certified programming). I took a few classes in those areas which were really fun and reasonably challenging. I also started research, which was boring, but it was only the first semester and I wasn't working on my own project.

Then I got an offer to co-found a startup that I couldn't refuse, so I left my Ph.D program after one semester and went to do startups.

While I was reasonably happy with my life in academia for the time I was there, I have been generally (though not always) more self-satisfied with my work in startups. I currently expect to make a greater impact with my work on startups. The only way I could imagine work in academia being more impactful is if I were working on a research project whose fruits I expected to be directly used by people doing impactful work (top candidate would be making a certified programming language that could be used to build and prove a Friendly AI correct). I could definitely see myself heading in that direction but progress felt very slow -- that even with very good use of time in my PhD program, I am not sure would have advanced my skills enough to do a good job at succeeding at such a task. Whereas I feel like I'm making really fast progress towards startups that will succeed.