The value depends on the trip.

I have a 30 hour journey tomorrow, SAN-IAD-GRU-GIG...7 hour layover at IAD. I have no useful status on United, I am an American Airlines flier. I ponied up for Economy + on the IAD-GIG segment (10 hours), but only because F was sold out. I also paid out of pocket for F on the SAN-IAD portion. It's 5 hours.

Yes, it's domestic F on United. But consider, looking down the barrel of a 30 hour door to door trip, ANY segment I can make more comfortable is worth the money. I don't even care about the priority lines or bags or drinks. I am thinking about getting as comfortable as possible when I can to make myself resemble a human by the time I get to the end door in Rio.

However, if I were just making a 2 hour run domestically, no way would I pony up the money and buy up. I may chase a miles UG, but I don't care much on a light trip. I can tolerate maybe 3 hours tops in coach. After that, it's got to be at least J.