I did it last spring. I was against the upgrade. The lady said I couldn't understand until I tried it, and (to my mild embarrassment) paid for our upgrade as a gift. This was SFO-MAD, on Swiss, open-jaw, returning from Zurich almost a month later. The round-trip upgrade was less than $2000 over premium economy.

It was 100% worth it.

The price, on a trip that long, wasn't really noticable. The value was. What you get for the money is about the same as you would pay to eat lunch, dinner and breakfast at a nice restaurant (we don't drink, but drinks included), along with a night at an expensive hotel.

This is exactly what we received.

We arrived early, had a three-course meal at the (stunning) lounge, we were served fancy soda as soon as we boarded, we were treated kindly and attentively for 12 hours, We were served two more very attractive meals (on ceramic plates with real cutlery) we slept comfortably in the lay-flat seats, and arrived refreshed and very happy.

Added to that basic summary are uncountable little details, like good soap and facial refreshing spray in the bathroom, little swiss chocolates, etc., that made the experience far more memorable and enjoyable.

I'm not set on flying that way from now on. However, I do now consider it to be a reasonable, attractive indulgence, rather than an obvious waste of money.