I think the OPs question was designed to get some interesting feedback and therefore was sufficient, but I would have phrased it differently: Was the AI in GC2 good enough to provide an enjoyable experience for the gamer? That's what a game is supposed to provide, in my view.
Suppose one could create an AI that was as good in GalCiv as Deep Blue is at chess. Well, that would be interesting to see, but not much use to me as a gamer trying to have an enjoyable experience. I would just be seeing how long I could last before I got crushed. A game is an entertainment vehicle, not a vehicle for measuring one's tolerance to humiliation.
So was the AI in GC2 good enough to provide me with entertainment? I would say definitely yes, though hardly perfect. It certainly was at least as good as in any other 4x I played, and I have played a lot of them. I will break it down into what I think are the key components.
Does the AI build itself up sensibly? Fairly well, but the Community Update currently in play was prompted largely (if I am not mistaken) by the problem that the AI (in TotA) was not using its tech tree well. or perhaps I should rephrase and say it couldn't use its tech tree well. My experience suggests that it does much better in the CU.
Does the environment one plays in seem alive? That is, are the AI doing things that appear sensible, do the AI factions interact with each other, making friends and enemies, fighting wars or making alliances? It seemed that way to me, but one would have to go into debug mode and study to really tell. But all I care from an enjoyment standpoint is whether it "feels" ok to me, and I would say, yes, it did. Some have said they want the AI to recognize who the human player is and play against him. That doesn't feel right at all to me. They should behave "normally" according to their traits and respond to objective threats and opportunities, not automatically dogpile on the player. That would feel very wrong, but its just me.
Here I need to comment about the constant discussion about the AI "cheating". I have a different view I suppose --- what I care about is what do I see and how does it feel? What's under the hood, well, that's for the garage mechanics and engineers to argue about. How does the car feel on the road is all I care about while playing. There are some sorts of AI operations that don't feel right and look artificial --- for example, units suddenly spawning from nowhere, clearly without having to be built or moved into position, etc. That's bad, but not because the AI is "cheating" but because it "feels" wrong. To me, anyway. Didn't see that in GC2.
I remember a game of Civ4 in which I thought I was doing ok against the Chinese, but after I penetrated their frontier I was faced with a stack of several dozen units. I just about fell off my chair. It was mid-game and though it was a very nasty surprise, it didn't feel like an exploit. After taking a deep breath and doing some thinking I managed to figure out a way to wear down that stack and eventually I won the game. Question is, were they able to build this stack because they were "cheating" or was it just good organizational planning, etc,? Don't know, don't care - except in an academic sense. It was sufficient that it seemed plausible at the time. I think at that difficulty level they had some production advantages, etc., which one could look up, but I didn't. I had a fun game, though I was "moderately" stressed for a while. I don't play a game to analyze its mechanics, even if that may be interesting in another context.
Does it avoid immersion breaking mistakes, of omission or commission? I think GC2 was fairly opportunistic if I left an "opening". Current example: A few weeks ago I was about to attack an AI colony in GC3 but stupidly left a transport unattended near a planet that was defended. I think I had thought that it wasn't, and wanted to get the transport close enough to invade the next turn. Oh well. The AI could have sent out his fleet and destroyed the transport, but just watched. His fleet could never have beaten my main fleet but he could have delayed the invasion many turns. (Beta 4 I guess, not sure). That's the sort of thing that the AI should jump on; I don't remember getting away with that kind of sloppiness in GC2. And I am not sure I could get away with it now in 1.03.
Does the AI organize attacks with a purpose and try to carry them out? Yes, I think so in GC2, though most times they are not the best plans, that's not critical. A bad plan is better than no plan. Heck, I would say most casual players don't often actually compose a plan of any depth. That's the first thing I see, say, on Civ Fanatics as to how to raise your game level. Make plans. Why is Marbozir so good? His plans, made very quickly it seems in his let's plays, are quite deep and generally spot on.
And lastly and most difficult, does the AI, seeing a threat, organize against the threat by trying to use cooperation with other AIs or with the player? That's a tough one. I am not sure that really worked so well, but I don't have a clear picture. What I do know because it is more recent is that in GC3 I have been asked by the AI many times to declare war on someone. This points in the right direction, but so far I have had the feeling that the AI really isn't as threatened as it claimed and that it doesn't follow through cooperatively against its "enemy". Rather, I wind up conquering the opponent, while the original AI makes peace! Well, it has a ways to go.
One last overall comment about AI. A lot of the discussion seems to revolve around how difficult the AI makes the game for the human player. I translate this to mean, how hard is it to win, and how much "help" does the AI need to make it challenging for the human player to win? Well, as in RPGs, there is another point of view. Does the game have to be about winning and nothing else? You can play Skyrim to finish the main quest, or you can role play, bringing an entirely different mindset, and with the objective being the journey and there not needing to be a destination at all. Heck, the Metaverse is broken anyway because of the blatant exploits people are using to run up scores, so who cares? I posted earlier about bringing some more role playing types of play into GC3, but of course, as always, that is just me, and the branch I am sitting out on seems to generally be pretty thin and pretty lonely.
My 3c. I am charging more because I am on a fixed income.