Atlanta as a major metro area has lagged severely behind in developing a comprehensive mass transit plan. It's due in part to the government structure we have. There is 13-16 counties in the Metro Atlanta area, each with their own agenda and plans. Without a central business district whereby a majority of the people are headed to and from one point, it makes a plan a challenge.
That said, the area will vote on a transportation tax come this year that will start to make some baby steps in getting somethings moving. It's severely needed to keep Atlanta competitive with cities like Dallas, Portland, Houston and Charlotte that have transportation systems already in place. ATL is about 20 years behind schedule.
That is a prime reason to consider an Intown Atlanta neighborhood if at all possible. You are closer to the business centers of Midtown, Downtown, and Buckhead. Even if you work in places like The Galleria or Perimeter your commute (by car of course) will be easier.
MARTA does have train service from the Airport North to either Doraville or North Springs. The challenge many face is once you step off the train, how do you get anywhere from there? But many people do take bus service to say the Doraville station from Gwinnett County, and then MARTA into the city. As others have said, it becomes a lengthy commute with the bus, then transfer to MARTA, and then possibly another bus ride.
Unless you live and work in Buckhead, Midtown or Downtown, you'll be in your car.