We recently flew via Atlanta to Tampa with Delta. I actually quite like the Delta flights - the staff are laid back, and it is very relaxed.
We had a couple of issues with not being able to check in online or the day before for a 9am flight, and they have changed their luggage requirements to no single piece can weight more than 50lb (it was an issue because they gave us the wrong information beforehand and we ended up repacking at check in ). Also, coming home, our Tampa - Atlanta leg was cancelled and we hadn't been booked onto a replacement flight, meaning we had a delay whilst they tried to sort it out, and eventually rebooked us onto a Cincinnati flight - it worked out in the end, but we are still in some dispute as to whether it was a Delta mistake (more likely), or whether Airline Newtork who we booked the flights through were responsible. It was the kind of hassle your don't need at the end of a holiday.
I have to say, despite these troubles, I would fly them again. They were significantly cheaper for us, they seem to discount child fares more, and they don't charge a lot more for flying on weekends. The flight itself was very pleasant - seat back tv, decent food. Just be aware that like most American airlines these days, it's $5 per alcoholic drink.