played a couple hours with the expansion today. it's definitely better, but I haven't been far enough into some of the mechanics to be too specific yet. took me a while before I established an embassy somewhere, and I haven't founded a religion yet. I'll keep updating as I figure it out.
Game seems to run smoother in general. I have everything on max settings and the game is chugging along nicely.
Combat AI does not seem improved. I held off an attack of 7 units and a Great General with an archer, warrior, and spearman + 1 city, and the city was never in any danger of being taken over.
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http://www.youtube.com/watch?v=npHDxSvwCE0