I usually fly in & out of Atlanta when I go back to the US to visit. It's not too bad usually. Pretty much any large airport tends to have flight backups & delays to some extent as the day goes on. Word to the wise: if you can get an early flight, do. Yeah, it sucks getting up in the wee hours of the morning to check in early enough & catch a 7 am flight or earlier, but a flight at that time is a) less likely to be as crowded and b) much more likely to be on time (unless bad weather intervenes which no one can control). As it gets later in the day, if there are delays - then those delays affect the flights scheduled after them, and it all starts to stack up on itself, and so on...
Case in point, on our recent US trip - I was dreading that we were flying on the Sunday after Thanksgiving -- typically one of the busiest days for air travel in the entire year. Yet our 7 am flight from Atlanta to Tampa was far from crowded & ran perfectly on time (no bad weather)! It was so smooth that you wouldn't have even thought it was one of the year's busiest travel days. Yet on the news that night -- there was all the usual hoopla about how bad it was at all the airports (to be expected, of course & there was some bad weather up north I think), but we saw none of that whatsoever.