I switched from DL to AA a few years ago and am very happy. MRTC is the most customer-friendly move any airline has made--though ironically I get upgraded on AA much more than DL (where the coach seats are barely tolerable).

I will continue to give as much business possible to AA but am trying to maintain some status on DL also--to protect my old Frequeent Flyer mile rights and also because, though I despise the SGB mentality as much as anyone I do believe it will keep DL in business long after other airlines have failed.

So my strategy: AA for love; DL for security.