A lot of high quality content, but a few grating flaws as well.
Unlike most 4X games these days, Civilization IV doesn't pretend to have some ridiculous campaign that has no real direction. You just set up a game and go, it can be a simple "Play Now!" setup, a custom game setup, or a mod game thanks to Civ IV's extensive mod support. Heck, the developers included quite a few mods out of the box.
You can pick from a large selection of civilizations, each with one or more leaders that determine what your bonuses will be in the game. It's disappointing that some civilizations only have one leader, but since leaders are essentially just a grouping of three bonuses it doesn't really matter all that much. Once you're in the game it follows a pretty typical 4X style, you build cities, build up a military to defend or conquer with, and meet the other civilizations that you share the world with, then either negotiate with or destroy them. Or be destroyed by them, whatever strikes your fancy. You go through multiple eras by performing research, you can go from "Ancient" to "Future", but sadly there's not really any cool futuristic technology present in the vanilla game.
Winning in Civilization IV can be easy or challenging depending on what you're going for. You can win by either have the highest score by 2050, spreading your culture throughout most of the world, the old fashioned command and conquer style, or by being the first to build and launch a space shuttle. The best part about winning or losing in Civ IV is that the game doesn't have to end even after the victor is determined, there's an appropriately named "Just... one... more... turn..." button that lets you keep playing past the so called "end" of that particular game. Although without futuristic tech this seems a bit pointless to me, what's the chances of humanity still using modern technology by 2500?
Visually Civilization IV is pretty good for a 4X turn based game. Each incarnation of Earth is very detailed, and surprisingly so are your cities and units. Every time you build a new structure in a city, it will expand and you'll probably be able to see that structure. Audio wise the game is nothing special, the vanilla Civilization IV main menu theme is hauntingly beautiful, but that's it. Civ IV features voice acting, but it's more annoying than anything. I can only tolerate hearing Leonard Nimoy's research quoting so many times before I mute the speech.
Civilization IV is a superb game, at least with Beyond the Sword. It does tend to hold your hand if you don't know how to turn off "Sid's Tips" (Pro tip: Options menu), but it's a fairly entertaining turn based strategy game that has a massive scale. Even the largest possible Earth incarnation feels a bit small compared to other games in the genre, though; even Civ IV's predecessors. It's not a perfect game by any means, but it's definitely the best game in the series so far; and one of the best games in the 4X sub-genre.
+Addictive "One more turn" gameplay
+A good deal of depth
+Excellent variety
+Very customizable
+Ability to keep playing even after victory or defeat
-Lack of futuristic technology or units in vanilla game
-Somewhat cramped game world even on the largest setting
-Leonerd Nimoy and Sid Meier's speech narratives get irritating after a while