My DH and I flew Delta last month from Milwaukee to Orlando. We were originally booked on Midwest Airlines, who was bought by Frontier, and then we were transferred to Delta.
We were not charged for picking our seats, but our flight times were changed two or three times, but never by much.
On our first flight to Orlando (we flew through Atlanta) someone at the back of the plane let out a large yell. While I was freaking out (I am a bad flier), it turns out a person in the back of the plane had a seizure. Delta did a great job reacting very quickly. Fortunately there was a doctor on board, and the person was taken care of properly! Delta did a great job with this, and it made me feel good knowing how quickly and appropriately they could act.