Ah, Civilization IV:BTS, the game that stole so much of my life. Civ IV is a classic. After some of the frustrations with Civilization III following the majesty of Civ II, IV felt like a return to form for the series. It has a lot in common with III, of course, but it made the game feel epic again. It also resolved some of the awkward controls the series was known for, and added a few intriguing features. In particular, religion, corporations, and civics (borrowed from Alpha Centauri).
What BTS gets right is amazing. There is a lot of depth to the decisions being made, and it can be a complex game if you want it to be. The civic system lets you fine tune your own custom government (a theocratic police state with a free market? Go right ahead!), with the rewards and penalties mixed, like in the classic Civs. The diplomacy is vital to the game, and the AI's opinions of you actually matter, and can be influenced.
Where it gets it wrong tends to be the tedium and the stacking mechanic. Civ III and IV suffer from a design oversight that became a mechanic, called 'stacks of doom" in which not only could an army be overpowered by playing upwards of 30-40 units in one tile, but such play became essential to win. The city attacking and defending mechanics also leave something to be desired, as they are intended for stacks of doom. The gameplay can be tedious compared to Civ V, many turns can pass without anything happening.
But it's also more satisfying when you get to victory in this game. And there are more victory options in Civ IV: BTS than any other Civ game, including early game victories such as religious.
This game was so good in its time that even today the Civ fans are divided between Civ IV and V, with many fans preferring this game over the new version. I do consider it a matter of preference, as I play both.
Last of all, certainly not least, Leonard Nimoy narrates this game in a Leonard Nimoy fashion. It fits the game perfectly.