In my experience from flying:
- Short haul business class makes very little difference from economy class. Plus as it's only for a few hours it really isn't worth it if you are paying for yourself.
- Long haul first class / business class is a much more comfortable way to travel than economy. In particular if you are flying overnight you get the flat bed, all the leg room you need, and your own little space all to yourself to move around in.
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.