Yeah, Civ is like a crack addiction...without all that prostituting yourself and all!
I've played quite a bit of Warcraft and C&C, but they just got old to me. I never played more than a few games of each just because it seemed like the same old, same old.
CivIII (Conquests) is an entirely different beast. As has been mentioned, it is turn based, so you get to think out (plan) your moves, even having to think a few turns ahead (like chess).
You can play the regular game, customize it to your liking, or, if those have bored you, play the scenarios within the expansion pack (Conquests). Then, if that isn't a challenge enough, play the many player-built scenarios that you can download *free* right here at CFC!!
Personally, I like the war aspect, so every other CivIII game I play I'll turn off the other winning options to focus it towards war. By the same token, if war is not your thing, you can customize the game (really easy) to focus towards science and/or diplomatic victories.
Awesome game! Definitely one of my all-time great games EVER!
