It can be a great deal if someone else (your company?) is paying. Otherwise sitting in F is still at least $5000 more SFO-FRA. That translates to about $250 an hour to sit in an F seat instead of a Y (or upgraded C) seat. F seat worth $250 an hour of your money? Nah. Think of the excellent suite in FRA or SFO you can get for several days or a week for that $5000. Worth a lot more than an F seat for a few hours.