I think the value that each individual puts on it, as with most things, 'depends'.

Because I fly regularly I am entitled to 8 upgrades a year on any route provided there aren't more than 3 segments so I can fly more or less anywhere in the world in First/Business without paying a penny more. I also have more frequent flyer miles than I'll ever need so I could avoid economy every time I travel by using those.

Because the 8 allowance doesn't cover all my needs I am selective on when I use them. To me, upgrading on night flights is much more valuable than on a day flight. As all UK-US flights are day flights I tend to use them for the overnight returns. That also means that I don't have to pay the extra rip-off £60 UK Departure Tax. An overnight Los Angeles-London in economy is brutal so I upgrade to Business on those but a Manchester-New York 7 hour day flight is perfectly acceptable so I don't waste my upgrades on those. I know many frequent flyers who take the same approach if they're paying for their own ticket. There are of course a few people who are 'in the know' and can get free upgrades and special fares - I've met quite a few over the years although, curiously, always in the Economy cabin.

I have never flow Premium Economy in my life because I think it's very little different to Economy. I have bought the equivalent BA World Traveller Plus tickets but only because I can easily upgrade those to Club with miles.

My own opinion is if you can't run to Business or First then save the money to spend on the ground. YMMV of course.