For a year and a half I was doing LHR<->SFO every 6 weeks or so. I wouldn't say I enjoyed it much, but it soon became a weird sport to optimize everything. I got it down to ~16 hours door to door, by e.g. ensuring I never needed to check anything in and trying out several options on either end of the flight. I maximised my upgrades by carefully picking flights, and so rapidly got up the tiers of the reward program (I was flying United, because while their standard was low compared to e.g. BA or Virgin, their reward program was far more generous, so if flying often it was a better experience back then). I found great headphones, sleeping masks, pillows. At that point it was close to enjoyable.
But that was with transatlantic flights. Having experienced domestic US flights, I can't imagine doing that regularly. Difference even within the same carriers at least used to be awful. To the point where I'd insist on direct flights over stretching my legs on the east coast because the domestic flights were so awful.
