I personally struggled to beat the AI for the first 2 months or so. So its reasonably good. I lost about a dozen+ games, some very, very badly.
Of course now I win with my eyes closed and hands tied behind my back, I hit the keys with my nose...
I could not say the same about alot of other strategy games, which I have been able to walk over after only a few days play.
So overall the AI is reasonable.
Exploring the game design, different nations, strategies etc... will still keep you playing long after you have mastered impossible difficulty.
Also I am currently getting a good, challenging SP game by designing the AI's gods for them, doing there first 10 turns to get them off to a good start, well handicapping myself by not moving out of my home province for the first 10 turns. Produces a challenging game for more experienced players.