Back in the mid 90's DL had fairly good service; phone service for reservations was very pleasant and helpful, the ground staff was experienced and there was actually a person who gave a c..p about your needs. In the late 90's and early 00's DL went downhill: poor customer service, staff on the ground that could care less and on board staff that made you feel like you are bothering them; I stopped flying DL in 2002-2003 and have been flying CO exclusively with AF, KLM (Skyteam days) and UA now.
I had the opportunity to fly MCO-ATL and ATL-SFO last weekend;
It brought back terrible memories and I suddenly remembered why I left DL and why I disliked it so much; the ground staff at ATL is horrible; the self-checkin kiosks work well if you don't need any help; if you have any questions or want a change then you are out of luck; The flight attendants were OK on the ATL-SFO and actually nice on the MCO-ATL leg but cannot be compared to CO;

So, I am not sad CO left Skyteam even though I miss the non-stops to CDG and AMS from SFO. I think it will change now with UA which is not nearly as user friendly as CO; I also think DL will be successful just because they are a 750 lb gorilla (CO - sorry UA - will be the next 800 lb gorilla).