I joined EC in October, after years as EXP with AA and lifetime PLT.
The primary reason was that I was unable to fly until October (little problem with cancer) and could not qualify for EXP during 2004. Then AA came out with a lit of little irritations, like a $250 fee to upgrade with miles - and that's a one way fee.
With EC I made Silver with one RTW and, to honest, prefer GA J seats to AA's coffins in F. I also appreciate the free lounge membership, compared to paying for the Admiral's Club.
The final benefit is that BA allows me to plug in my autopap (for sleep apnea) when I want to sleep. AA is still in the prop era with paps - probably nervous lawyers. That means BA for all crossings of the Atlantic.
I'll still use AA domestically (almost 100 upgrade stickers on file), but the EC is looking pretty good for me.