Our long, over 30 hour layovers are in Buckhead. Very nice area. Great shopping, there's a movie theater and even a major supermarket for those on a budget. We're staying at the Crown Plaza which is nice and we have stayed at the Sheraton Buckhead. The only downside is the traffic to the airport.
If I were staying downtown or at the airport, I'd take MARTA for the day to Buckhead.