Actually, I used to be seriously addicted to Civilization I, less so to Civilization II and simply not at all to Civilization III. Even with all of the expansion packs for Civ III, I think I only played the game twice and only an hour or so each time. It was just so boring somehow, I'm not really sure how to describe it.
Civilization IV on the other hand is, at least for me, jaw droppingly wickedly sweet!!
It just seems to have all of the little touches that sweep you off your feet and compells you to keep playing. The music, especially the main menu music is so outstanding that I actually emailed the mp3 file for it to my office address just so I could listen to it at work!
The graphics revamp is gorgeous and the world is really beautiful. Unit functionality seems to work more like Civ I, especially with the fact that catapults, cannons, etc can now attack on their own as a unit and not be relegated to a support only role. Diplomacy is reliable now!!!! You can actually form lasting friendships with NPC civs and not have them spaz out on you and attack you. They will frequently seek you out for treaties, trades, requests, etc without you having to be the one to initiate it each time. Oh and they got Leonard Nimoy to do the voice over work!!!! My god that was great!!!! Hearing him read the technology quotes and such makes me feel like I'm listening to Mysteries of the Bible that he used to host. It's just so damn cool!
Obviously there are still WAY more features than I can name in a single post. The bottom line though is that this game is very polished with no bugs apparent to the player. This is DEFINATELY a "just one more turn honey" game and you will spend late nights with it.
If you enjoy Civilization games, this is by far the best one ever made... and that truly is saying a lot!!