I have a one way flight coming up next month that was going for about $1200 via SEA in F when I was booking but I went with C+ instead for $308 (MUCH cheaper F fares were/are available via other hubs but I wanted to connect in SEA for various reasons). I just checked my trip this morning and saw this offer pop up when I selected a seat on the SEA-ATL segment:
I wouldn't even consider the upgrade on the short hop up to SEA but the ATL segment has me thinking about it.