I was at ATL last weekend and didn't have any problems. I spent three days there and was only approached one time by a parking garage guard who told me that she didn't mind if I take photos, but that her manager says it's not allowed. When I told her I would be happy to speak to the manager, she said ok, but the manager never showed up. While inside, I wasn't bothered at all, and even was helped by a gate agent when I asked him when a certain aircraft was leaving so I could photograph it.
From outside, I recommend the north parking garage. From there, you can get good views of takeoffs or landings and if you are taking photos, you will be facing away from the sun.
Inside, there are windows all over where you may be able to catch good views. If you are taking pictures, the tint in the windows may affect your results. Delta is pretty active there, so an unoccupied gate will only stay that way for a short while. I found that for afternoon views of the International Concourse E, Concourse D is good because the non-Delta gates there seem to be not as heavily used. I spent quite a bit of time at a US Airways gate there. There are also windows down at the end of Concourses C and D from which you can view the runway.
I hope this helps. Have a great time on the trip.