I think I'd pay $600 for a PS upgrade on a 10-hour flight, unless it were a middle seat (unless it were the bulkhead row). But, this sort of decision is really dependent on your own financial situation. The 26-year old me would not have been so casual about spending $600 on such an indulgence. The 54-year old me, much more financially comfortable, would probably not think too much about doing it.

In general, I think a lot of FTers are too negative on Delta's Premium Select product. I happen to be a big fan of PS, especially for flights whose entire duration will be during origin waking hours . . . I mean, what's the point of D1 if you're not going to be sleeping for a extended period of time? If it's about the food and wine, well, I can have a fancy (and better) meal on the ground for <5% of the price of a D1 ticket. If you're awake and sitting up, I think PS provides a level of comfort almost equivalent to D1, and it's less of a hassle to get in and out of the seat.