I would absolutely never pay for business/first class in cash. Even 24+ hours of discomfort is not worth it to me (I'd sleep in the street for a night if somebody offered me $500 to do so, so paying a bigger difference than that just to sleep in a somewhat more comfortable chair is certainly not something I'm going to do). However, for award travel, business class is sometimes only negligibly more. I can also understand why companies pay for it (so that their employee does not lose a day of productivity on arrival). I have flown business class several times on awards or when someone else was paying.

Note: the extra comfort of business class is worth about $5 per hour to me. Thus the only time I have voluntarily paid for something other than economy was on the Trans-Siberian railway from Moscow to Beijing, where a first class ticket cost $250 more and allowed one to shower during the 133-hour trip.