Machine Learning

I left mine 3 years in, was getting my PhD in CS (ML/statistics focus) at a (the?) top school for it. Lots of factors including depression, RSIs, and dissatisfaction with inevitable job prospects. I loved research and being surrounded by brilliant people, but felt disheartened that my passions outside of school were getting severely deprioritized. I know it's not strictly true across the board, but from my POV, the successful people in that field eat, sleep and breath their research, and it's essentially their life's purpose. I never felt that way, and I didn't really feel the need to compete with people who obviously were so obsessively passionate about their research.

The thing that finally allowed me to leave is that I started a company during my PhD that was completely unrelated to CS that became profitable very quickly. It was too much to balance the PhD and the company, so I left to do the company full time. I recognize that's a hard situation to manufacture, but for what it's worth, I am so insanely happy with my life and would never even consider going back. I have zero regrets -- the only thing I miss is being surrounded by a high concentration of super smart people.