You have to be pretty blind not see the positive changes to the AI. Diplomacy specifically has been beefed up significantly and while the tactical AI is far from stellar, it's a huge improvement over vanilla.
Try as I might, I failed to see a significant enhancement to the strategic AI, and an even less challenging tactical AI.
For example, the AI is absolutely terrible at conducting warfare, strategically and tactically. On several occasions I was surprise attacked by armies of units that stood no match for my basic defenses (mohawk warriors against castled cities with artillery or knights against great war infantry). Once the AI had lost the majority of its units, it sued for peace with all its gold, luxuries, and resources. This happened several times, across different games and opponents. This was on or above emperor difficulty as well.
When an opponent did attack my cities with units capable of capturing them, more often than not, they completely blew their opportunities with ridiculous moves or premature surrender.
It also appears that AI does not compete on the same grounds as the player, much like Civ 5. In order to keep the game somewhat challenging, the AI is not made smarter, but is simply given advantages (read: it cheats).
Though perhaps, I may be "pretty blind" as I am disagreeing with the post above me - that, or I'm not affiliated with the publisher/developers.
I wonder if we are playing the same expansion here. Where in vanilla the AI attacked with archers first, now it arranges them properly - melee units, then ranged, siege out in the back. Where in vanilla the AI paraded around with its catapults, now it actually tries to protect them. Where in vanilla it threw all its units at you, now it actually keeps a considerable army back home to defend. Where in vanilla the AI barely built any ships, now it's capable of staging some very big naval invasions. Where in vanilla it never retreated troops to heal, now it does so.
It's far from perfect and every now and then it would make something pretty stupid, like leaving a GG out in the open, running around your border with an unprotected worker or shuffling units around instead of attacking your undefended city, but to say that there hasn't been a considerable improvement is indeed somewhere along the lines of being blind. Or of course not having played enough to take notice.
And suggesting that someone who did notice these change is in any way affiliated with 2K or Firaxis is very childish. It's a stupid and tired argument that we both know is basically never true. I live 6000 km away from Firaxis for Christ's sake, look at my name (?!) I like the expansion, so I must be in cahoots with the developer? Really? I mean, really?!
If you don't believe me (since I work for Firaxis), there are plenty of testimonies around the net, including on one of the most prominent Civ fan sites: http://forums.civfanatics.com/showthread.php?p=11588837&langid=1 Or maybe all the CFC members who say the AI has taken a step forward are working for Firaxis as well? An international conspiracy of some sort?