I've had quite a few epic battles, and was quite impressed with the a.i.
Granted, I'm no superstar player, but I've had the AI almost ovverun an upgraded starbase, but once the repair rate of the starbase exceeded the incoming damage, it was all over. The ai didn't retreat as that point, but it was still close.
I've seen their fleets move out of the Marza death circle...as much as I've seen them stay in and get nailed.
On the poor side, I've seen the AI attack with Lone Cap Ship Vs. Starbase.
I've found on large maps , at least as TEC, it's easier. I just send volleys of Cannon shots aginst planets to keep them off balance until the fleet takes the planets one-by-one.
I've had far more fun, (and nailbiting) on smaller maps when I'm bound to encounter the enemy sooner rather than later. The small fleet skirmishes of 1-2 Caps plus small fleet vs 1-2 Caps plus small fleet as far more intense.
I also ty out diffferent capital ships to see how they play out in those small scale battles.
One of my most fun battles was with a Rapture (or more precisely it was the 2 points of Vengeance) that destroyed 2 enemy cap ships. I don't think any other single cap I've tried was able to do that...
I'm sure the AI could always be improved/tweaked, but still...overall kudos to Stardock.