Mac's biggest strength, at least if you happen to live and/or work around Midtown, is its convenient location (on West Peachtree just south of 10th). Its beer selection is decent, its wine selection pretty good, and its liquor selection quite good.
Prices are acceptable---they're cheaper than you'd pay at Kroger or Publix, but more than you'd pay at a liquor store outside of Midtown. If you're stocking up, this isn't the place to go, but if you're picking up a six-pack on the way to a friend's place, there's no reason to drive somewhere further just to save $0.50.
There's usually plenty of parking in a small lot behind the store, which you can enter from Peachtree Place (a small street one block south of 10th---what would be 9th if it followed the numbering scheme).