I think a lot depends on the extra cost overall. We recently flew premium on TUI (2 adults and 2 young teens) and we thought it was worth it on the 10 hour flight, but it did end up being about £2k more on the overall cost. The benefits for us were:
•fast check in (although the normal line looked like it moving) and fast pass security (but that’s cheap to buy as an extra)
•lounge access at the departure airport (we’d usually eat breakfast at the airport, so that was a small saving)
•seats were spacious and comfortable. They were big enough that I didn’t feel confined or irritated if the person in front reclined, and I didn’t feel guilty when I wanted to recline. This was important for me because i feel stressed about reclining on someone and causing them discomfort.
•service was attentive throughout the flight, and the cabin crew were lovely.
•pretty constant bar service (not sure what non premium is like with TUI though)
•food was marginally better than your usual plane food, with better cutlery
•rituals amenities kit (on the outbound flight only, so keep hold of it!) was a nice touch
•bags first off
•you get to board first
The main benefit for us was the bigger seat, meaning we were more rested after the flight, and the jet lag wasn’t quite as bad. One of my dc is a very nervous flier, so part of us booking it was to make it a better experience for him, and it worked (no panicking and our most recent non-premium flight was easier for him after having a good experience).
worth noting that the entertainment is the same.
I did a bit of research beforehand, and one tip that kept coming up was to try to avoid rows 8&9 - if they change the aircraft to a smaller one, these rows get bumped to economy (although this didn’t happen on our flight)
on balance, I would do it again if the price wasn’t too prohibitive.