Just had really poor experience with DL on a PHX-LAX-KOA ticket. DL totally unreliable and phone agents were terrible - first two refused to rebook us in F on an OAL when DL PHX-LAX was delayed several hours past LAX departure time. Ultimately wound up on UA for three of the four flights (and lost a full day of vacation). Service on the single DL segment we actually flew (KOA-LAX) was abysmal.

But unless you're SEA based, as an actual frequent flyer, AS alone is not enough. I have to mix and match AS with UA and AA to get where I'm going. AA is the natural partner due to the reciprocal benefits. It also has the best premium t-con product.