You've basically got to fly a lot AND fly full fare refundable tickets (easily twice as much as regular tickets). So it's basically only useful for business travelers who are on business.

Once I got on United Gold, I could pick weird flights (undesireable times, maybe the shittier airport) that would automatically upgrade a full ticket to first class. But a full fare economy is basically the same price as a non-refundable first class. So it only works if a business is picking up the tab.