As I do with all versions of Civilization, including the "lateral" versions such as "Alpha Centauri" and the non-Sid Meier version (I forget its name), I play it until the cows come home, for weeks and weeks. It simply does not loose its power to entrance. However, there does come a time in every version where parts of the game start to wear thin, such as when you outstrip all the AI opponents in tech and wonders acquisition (I always play on a relatively easy level) and its a matter of waiting until you can reach a culture or diplomatic victory. Both primary additions in this expansion add good interest to the middle of the game, with religion adding another dimension to early game strategy and country selection (Byzantines and Celts are especially good if you want to make the most of religion.) Diplomancy adds interest to the middle of the game, since it doesn't kick in until you reach the Renaissance, which is just about the time religion starts to lose its lustre.
As many people have noted, the religion addition is far, far better than the religion aspect of Civ III and Civ IV. It adds a new dimension and especially a new resource to enhance your quest for culture, productivity, money, and happiness, plus the new commodity, faith. If I had any regrets about the religion addition, it is that there is no bit reward, like a new victory condition, for succeeding in spreading one's religion. The only drawback of such a victory condition is that to be realistic, it would probably peak early in the Industrial age, and one would lose about half the game narrative. Well, one could make it difficult, with perhaps a 1 in 10 chance of attaining it before the advent of the Modern Age. Even if not a win, then maybe a big boost in culture.
The diplomacy addition is a neat addition, but it is less engaging than the religion addition. Like the religion addition, it puts a premium on getting a really fast start, to be the first to reach the Renaissance, when you get your first spy, and when you can start building spy deterrents such as constabularies early in the game. Both additions add annoyances which tend to spoil things if you like to "win big". It seems virtually impossible to prevent one of your competitors from stealing at least one technology, and short of going to war with an opponent, it seems almost impossible to keep an opponent from converting one of your cities. While stealing techs is reasonable, it seems entirely unlikely that by missionary efforts alone, one could convert a major city from one religion to another. It's like imagining Billy Graham would go to Medina and convert it from Islam to Baptist Christianity.
Combat units have been improved in several ways. I'm especially happy with the set of combat units which bridge the gap between the Civil War and WW II, such as the Gatling gun, Great War Infantry, Great War Bomber, Triplane, and Land Ship (early WW I tanks.) Defence seems to have been strengthened for the early game, but there is still a huge jump in advantage to the offense with the advent of artillery, the bomber, and the battleship, when attacking indirect fire units can stand off and damage a target, whiile remaining out of range (except for the bomber). Possibly the greatest single improvement in unit capabilities is the ability for sea units to attack and capture coastal cities. Oddly, while this capability may have been more common in ancient times, in the game time, it is easier after the arrival of privateers. (I just wish they would have kept the clever ruse of making the nationaltiy of privateers unknown, as long as you were only doing ship to ship combat.
Regarding diplomacy, I am still annoyed with how an AI country can denounce you and turn from friend to hostile for no apparent reason. There are simply no provocations apparent, except perhaps national character. The Aztecs and Alexander the Great, for example, seem to do it far more often than the relatively peaceful Arabians. But even Ghandi can get erratic and turn against you for no clear reason.
The designers seem to have made a point of coming close to parity between male and female leaders. There are several female leaders of civilizations where there are more prominant male leaders one could have chosen, such as Dido of Carthage in place of Hannibal and Maria Theresa in place of Emperor Franz Joseph of Austria. On the other hand, it is quite true that Isabella may have been the strongest Spanish leader, even stronger than her husband, Ferdinand. I do miss some of the more colorful leaders from earlier versions, such as Winston Churchill for England, Abraham Lincoln for the US, and Fredrick the Great for Germany.