One thing for sure for customer creature comfort on flights over 5 hours it is well worth the price
in miles upgrades round-to-its unless one has an affinity to travel compressed sardine class for savings.
Flights from the USA Mainland to Australia - 14 hours
USA to South Africa - 17 hours
USA to Europe - 6 minimum closer to 8 or 9
USA mainland to Hawaii - 5-6 hours
USA coast to coast 5-6 hours
Just some examples and you thought that train travel was a lark.
Oh, I don't know. I was always able to handle the 5-6 hour coast-to-coast flights in coach. When I started flying regularly, I thought I'd need meal service for even the 3 1/2 flights BWI-DEN or BWI-DFW or BWI-SAT and certainly for the coast-to-coast flights, but a few flights on post 9/11 legacy carriers and Southwest Airlines trained me to just suck it up and buy a sandwich at the airport before boarding.
What really surprised me was that I was able to tolerate the 13 hour flight from Dulles to Beijing (and back) riding Economy Plus in United. That's pretty basic service, the only thing Economy Plus gives you is a little more legroom. Of course, it helped that the flight wasn't full, and there were no people sitting next me. The food service was really bad -- made Amtrak flex dining look like the ultimate gourmet experience. In fact, the "breakfast" "omelet" (or whatever it was) prepared by the Beijing commissary and served before our arrival at Dulles was truly "mystery meat" and "vomit inducing" and "worth throwing out." Anyway, who needs gourmet food on the plane? I got to enjoy real authentic hot pot and Peking Duck on the ground in Beijing. On the other hand, it was open bar for decent microbrew beer and plonk, which helped me sleep a bit better on the long flight. They also had a decent selection of movies on the seat-back video screen -- I got to watch "Kedi" (a Turkish documentary about Istanbul's stray cats), "Wonder Woman," and "Close Encounters of the Third Kind."
When all this is over, and I get a chance to fly across the Pond to ride the trains of Europe, I'm not sure how much I would pay for upgrades. Premium Economy might be worth it, but I'm not sure about paying a $1,000 - $2,000 or more surcharge for first or business class for an 8-hour flight that is under $1,000 even in premium economy. Of course, I don't fly enough to get airline miles/points, so any upgrades for me are cash out of pocket.