Started w/ DL even though in US/AA mini-hub (DCA) - Amex helps - on time - IRROPS - dont mind connections in ATL/DTW/MSP - moved up slowly from nothing to FO now GM - each level, even basic no status, service always top notch.
What sealed it for me was how they handled a medical situation that came up 2 days prior to a trip with my wife in 12/2014 - call me a sap, but they way they handled themselves and each employee spoke to me, they have a customer for life. Now, I find myself on AA a bit more these days, but always check DL first.