My problem isn't even legroom so much as seat width. United has a 3-4-3 economy on some of its transcon flights, I was in one of the 3s and this huge guy (not fat just a brick shithouse) sits down next to me and it was impossible for me to sit without contorting (broad shoulders). Managed to snag an aisle on the 4 seat a row up that was open, then that guy boarded and sat down in my seat instead of making me switch back. Doubly surprising because I had what must have been the only open seat on the flight next to me by sitting in this guy's seat. That flight would have been torture, I'm still traumatized.