I'm an SFO-based DL PM. It worked great for me last year, since my flying to Europe would require a connection even if I were on *A. I like having the choice of flight options connecting through AMS and CDG. VS is nice but doesn't have onward connections from LHR (without a terminal transfer and all that). The biggest hole in their route map, for my needs, is the DC area. For those trips I've been taking nonstops on either UA or AS.

Delta runs a pretty nice and consistent operation. Their app is great, I've never had a problem with tight connections in DTW/MSP/ATL, and the inflight product is solid and consistent.

FWIW I looked at doing most of my travel on AS, but the lack of international options to Europe (just BA and Icelandair...) was the dealbreaker for me.