For me, I travel to a diversity of places and my company doesn't pay for biz class UNLESS I'm getting off the plane and going straight to a meeting, or the flight is longer than 12 hours (Japan, China, India, all of SE Asia). If, in a year, I hit Dublin / London / Paris / Tokyo / New Delhi / Shanghai, in addition to my domestic travel requirements (which are significant), a foriegn-flagged carrier makes no sense.
To hit all of those cities on one FF carrier, I'd have to connect internationally OR fly 7 different airlines (6 international destinations plus domestic), all of which would be in coach (except VIDP / RJAA / ZSPD), which sucks. Plus, I get no personal upside with the travel (frequent flier miles) that I can use or donate to a worthy cause.
I have had exceptional service on a number of airlines, including US flagged carriers. I've also had horrific service on each of those same carriers. In general, if you are a big carrier (flagged or other), it's going to be a crap shoot.
Cheers,
-Andrew
who's been on United, US Airways, Delta, JetBlue, American, Aer Lingus, British Airways, Lufthansa, Jet Airways, and Air France in the past 24 months