Livre d'Or

And if you have a family or long-term relationship, it's appallingly disruptive.

Or aging parents, or young children, or sick family members. Of course, these things can happen to anyone at any time (I did my MS in my early/mid-20s, during which my dad was diagnosed with Alzheimer's and then cancer and died, and the whole experience probably added a year to my degree--the only reason I didn't take a leave of absence was because I couldn't financially afford it), but the likelihood does go up for most people as they get older.

I suppose it's a tradeoff. I'm still trying to decided how much I regret grad school, but I'm glad I didn't go for a PhD.