In my experience from flying:
So if you are paying for it yourself then it comes down to how much money you have to spare!
If I had limitless resources I would always fly long-haul first class / business class (the difference between the two isn't huge in my view). But if it's costing $5,000 then effectively you're exchanging some other purchase for comfort on a twelve hour flight.
So if resources are scarce, in my view it's better to fly economy and spend the money saved on a great hotel, a fancy restaurant, a better car hire for the week, front row tickets for the show, or all the things that make a holiday memorable for you. The plane journey is soon forgotten.
I normally travel economy and struggle for comfort. i am tall and struggle to sit in the aeroplane seat. My knees bash against the seat in front and I am basically wedged. I know you pay for what you get so I decided to upgrade to business on my last long haul flight.
It was three times the price of economy and a fantastic flying experience. I had a bed and a bar and lots of extras. I enjoyed it but i wont pay that price again. Nothing is worth 3 x the price for a 12 hour air flight.
I have tried upper economy though and what a difference it makes. Your seat is more comfortable and you have a foot rest like a lazy boy chair making it slightly better. The price of this ticket is only about 30% more but is worth it. You also get given extras like the lounges before flights with free food and drink. They also allow more baggage weight with an extra 10 Kg's.
Out of economy, upper economy, business and first class the bargain is upper economy and the one to select for every trip.
image source traveller.com.au and airfrance.com
Traveling first class is just for the fun of being separated from the others. Nothing that special about it. Though if you have the money to give yourself such first class treatment, there's nothing wrong with it. At least you'll be placed in a separate compartment and given a first class treatment.