During my working years, it was always business class for any flight over 5 or 6 hours and, especially, if it was an overnight flight. The main benefit for me was being able to lie down flat and have some hope of sleeping. ...
My former employer had a blanket prohibition against "premium travel". But it was possible to concentrate flying itinerary on one airline family. I chose United (Star Alliance). United was never known for its generosity in bestowing "elite" status, or in redeeming points for upgrades. But after enough flying, upgrades to business class became a fairly common occurrence... maybe one time out of three. I never experienced international first class, but business class was... OK. I came to prefer older reclining seats to the "lie flat" mini-beds, because I am short yet very broad-shouldered. Shoulders are too broad to fit into the mini-beds, so I'd have to sleep on my side... not very effective. With the reclining seats, I'd stuff my arms under the lap-belt and hope that they not flail-out while sleeping.
Is it worth it? Well, in the good-old-days, those being circa 2005, cabin loads weren't 100%. As a semi-elite passenger, I could talk to the stewardess about getting relocated to an empty row. A row of 4 empty seats in the back of a 747 is more pleasant for sleeping, than a lie-flat seat up in business class. Today, with nearly 100% passenger loads, this is no longer possible. Business class is more prized, because coach-class has become execrably worse. Seats are narrower everywhere, exacerbating the shoulder spill-out problem.
If I were going to splurge on international travel today, I'd first research the seat width, whether it's in coach, "economy plus", business or first. Being able to fit comfortably in the seat, would be my first priority, never mind the food, the service, the ambience or the prestige.