Any other time, it'll be sort of crowded since it's a holiday and there's a bowl game going on, but certainly manageable. I've never had an issue using it before.
One year we were staying out in Buckhead, and we rode the Marta into downtown for New Year's eve. A friend was way to drunk so we moved towards the Marta entrance, saw the Peach Drop, and immediately walked under towards the train. We were back at our hotel in no time, because we avoided the massive crowds behind us.
Edit to add: Decatur is actually a nice little area. I stayed out there once and rode the Marta in for a Thrashers game.