I would say MARTA is safe, though you might get someone on the train asking for a dollar or find a homeless person sleeping in the back corner of a train car. MARTA Police are always around to discourage that, though. The neighborhoods between the airport and downtown aren't exactly scenic, but you fly through or above them at about 70mph. Strangely enough, the ritziest private school that all the extra-privileged kids attend is down there, and they all take the train up north to the wealthy neighborhoods to be picked up by their parents. I was always more bothered by the kids than the vagrants, as they were much louder and took space from adults trying to get home or to work. Since most of us are arriving on the weekend, that shouldn't be a problem.
...just don't call it "the" MARTA to a local.
William D. Richardson, CAP, MEP, TA
Director, Rocky Mountains Region
Member, Certification Administration Committee
Denver, Colorado