There are great 'natural history' museums, and there are great art, or science or other museums. I think this is the best museum I've seen that is just about the local area. It is incredible that you could spend a whole day just learning about the heritage of the south, of Georgia, and Atlanta. This museum includes a view of the city from past to present, starting in the early days when it was just a railroad link, up to today when it is the capital of the southeast with transport, commerce, and a great culutural center.
Civil war section is probably the best I've seen. Make sure to go out back and check out the mini-farm, and the beautiful mansion owned by one of the coke founders.
Pros: Comprehensive, Varied, Changing Exhibits
Cons: Location (no marta), some exhibits worn, more kid stuff