I dropped DL after I couldn't use my upgrades (U fares), and then they all disappeared on 12/31 (18 and 4 systemwides) with no compensation. Not a peep from them about a PM that stopped flying them.
I dropped NW after they refused to give me double mileage last fall saying that I had registered too late for the promo (even though I had screen shots saying I had registered in time).
I fly AA and AS most of the time and UA occasionally now.