My experience with the customer service agents over the phone and on Twitter has been nothing short of pleasant and professional. I've had some pretty gnarly problems, and they've solved them all – not always promptly, but always with a desire to get it right.
At the airport has been a hit or miss, but they're stretched thin. I get it.
Onboard, the FAs continue to provide excellent, professional service.
Compared to my recent experiences with LH and KL, DL is still at the top in my book.
I'm sorry you've had bad experiences.