Been playing on prince as Sumer. France AI expanded reasonably well and was more or less keeping up with my pace technology/culture/influence wise (he got to classical before me) - though he got fairly nice dirt and the torres de paine nearby. It had a small army (like 1 warrior and 3 slingers on turn ~50) - and I had been hunting barb encampments, so I had a fairly decent army by then. I declared war after I saw a french settler near my location, and while initially I didn't intend to take france down, I decided to do so. Some observations:
-Upgrading units is extremely expensive. Archer to crossbow was like 200 gp.
-Early on, without any kind of city defense, the attacker really has a big advantage. I basically razed the city outskirts unpunished while attacking it.
-The AI can play some tricks, but it sometimes makes bad decissions - like pulling his hard-built slinger out of the city to attack a unit within range. I unknowingly baited it, leaving the city undefended.
-Sieges are damn brutal. I had help from a small barbarian horde, who was passing by and between them and my units Ruan was considered sieged, and could take it in about 2-3 turns with only 2 units (an archer and a warrior).
-Movement is a bit frustrating - especially riverside jungles. The AI had a fairly hard time maneuvering around those...
-Barbarians are a bit excessive. Turn 20 or so, I had 2 cavalries and a horse archer at my capital's door. A slinger I had hard produced, another one I rush built and the policy that gives +5 vs barbs saved the day, but had I not had all those nifty prince bonuses from prince difficult I could have been in real trouble. They get units really fast. I believe that one of the biggest reasons why the AI had such poor army was the barb's aggressiveness.
In general, I feel the game might be a bit hard for the AI to play at lower difficulties - too many choices, and barb pressure is really tough on them. With some production bonuses it might be able to cope with barbs and keep a good army working.