Looking back over a short 7 year or so career I can't remember many projects where I can go: "Oh man, I'm so glad I spent all that time late at night on that project. It's really made a lasting difference in the world."
I'm sure there are some things that are worth spending a lot of overtime on; I'm sure there are ways to write software that will literally make a massive change in the way the world works. But most of the stuff that I see coming out of startups, most of the stuff that I've worked on in a wide variety of companies is stuff that ends up being rewritten soon, or changed or what have you.
One of my favorite CS professors was diagnosed with terminal cancer relatively early in life (late 50s, early 60s). He had another 10 or 15 years of teaching in him probably if he hadn't gotten sick. Towards the end of his fight with cancer, one of the other professors visited him and came back to us and said that he had been visiting the dying professor on a way to his daughter's flute recital. The dying professor looked at him when he mentioned the recital and said something like: "Good! More flute recitals! More ball games! Fewer papers! fewer conferences!"
I know that the time I spend away from work, particularly on my family – my relationship with my spouse, with my kids – ends up being the time that matters most in the long term.