I have had no issues with Delta out of PHX. Every flight has been on time and my luggage is always waiting for me when I get off the place. The employees I have encountered at Sky Harbor are always friendly and smiling.
The best time to fly out is on a Wednesday evening you have a better chance of the flight not being as full. Since they depart out of terminal 2 , it is less congested and easier than having to deal with American out of terminal 4.