Presumably, civ5 will have modding capability as strong as civ4, which will in theory allow anyone to remake the AI from the ground up.
As for the vote. I voted civ4+bts for now. I have only played 2 half games in civ5 on king up to now (i am currently waiting to get a new graphics adapter) so i have only seen a bit of the AI sadness (rome in war sending a string of single archers to my borders), and it's too early to tell how the economic part of the game will hold up to compared to 4, but i'm also worried about the apparently unit-heavy late-game-tech tree.
The flow of civ5 in what i've seen isn't to my taste. Turns to often have me attack barbarian boats with my cities, to the point where i have to attack some boat in 30-40% of overall turns. You have to click on the icons on the right to go there. At the same time, few exciting things happen per turn. Even so, when i was playing, there was some measure of "just one more turn" going on, but it felt less rewarding than any earlier titles.
Actually, it feels like doing somewhat boring work.
I'm astonished that people still vote for civ2 and even civ1, over civ4. Those games had no AI to speak of. Alpha Centauri, which to me was the best civ-game as far as flavor, ideas, immersion in the series if you count it, also suffered from AI.
Of the old titles, civ3 and civ4 are the only games that still pose a challenge to me, so in terms of replayability, they are the best. I like civ4 alot better than civ3.