Here's my assessment of what they have done to the existing civs:

Aztecs: early game boost, mid/late game nerf
British: boost
Chinese: tiny nerf
Japanese: small boost
Mayans: nerf on all bonuses
Romans: small boost
Russians: decent boost
Spanish: tiny nerf

Given the tiny nerf on Spain, I think they have more work to do. In the hands of an expert, Spain was the equal of Maya, and would dominate any other civ. They haven't done much to reduce it (yet).
posted by Tim at 8:12 AM