In attack, they're a lot better. In defense, they're still the same old, same old. That's my experience so far.
I'd like to comment on this. Only played a few games but I think this has a gameplay effect. In these games at a certain point some Civs had got the upper hand science and military wise. And what I noticed was that Civs were suddenly getting wiped of the table left and right.
So a long game with the usual wars, and then it tips somewhere, where the stronger AI is doing okay with it's attacks, while the weaker AI has littler means to play a defensive game.
So at a certain point some aggresive Civs suddenly take their continent in a matter of few turns.
Well I saw this in a few games and don't know if it's an unexpected consequence of the improved AI.
Devs should look at multiplay, like on the defensive people plop down a citadel and fortify a melee unit and try to use ranged around it.
Small things like these could enhance the AIs ability to defend itself.
I also see AI airforce suicide themselves, and planes seem to go down much faster, often on the first turn. Like 9 great war bombers lost attacking a city with a piece of artillery and a soldier around it.
AI still don't do intercept and airsweep, so these things just don't exist. They should improve this too.
Still also air combat is better. Just had a game where Attila overtook 3 French offshore cities with 3 Carriers full of bombers. Unstoppable to Napoleon. He did have the cities at 0 hp without any units to take them, but he got them in 5 turns or so.
So better, but still with a few eyesores.