Worth what? If you mean "Is it worth the difference between a discount economy fare and a paid F ticket," no way (at least to me). If you mean "Is it worth using upgrade credits for," on AA and DL (assuming you've paid a qualifying DL fare) I'd say probably yes. (I don't have enough exposure to UA's program to know how its upgrades work.) I like it for the space, the service, and the fact that (in my experience) you tend to meet more interesting seatmates, including one a few years back who has become a personal friend and a good client of my firm.

Breakfast, on the other hand, isn't a convincer for me. If you really want a good breakfast, fly AC. Their coach breakfast, the one time I had it a couple of years ago, was a tad better than any US first class breakfast I've ever had. I can only imagine what THEIR first-class breakfast must be! (I know, they don't fly that route ...) You can eat on the ground before you board for a lot less than an upgrade costs.