I've generally been happy with Delta's customer service of late. Both times I had IRROPs recently I got phone calls to help me navigate the situation (neither was a "meltdown" situation, which I have been fortunate to avoid). I get thanked for being a DM more than I need to be. Most FAs are pleasant and helpful (I've had a bad run with Pursers in the D1 cabin recently as the only exception). When I call in it is hit or miss as others have said--I don't get the sense that there is any Diamond Desk, but rather you get the next available agent quicker.