I became obsessed, probably border-lining on OCD, spending hours and hours just waging war on the AI's, then when I became more confident in my abilities skills, I went into Multiplayer. Now multi-player for Civ: IV is the same for Civ: V, making the adjustment to the new hexagonal system easier.
If you've played this game from Civilization I to Civilization V, you'll notice how slightly more complicated your war will be when your cities and units are surrounded by a team of 6 units. When I first tried this game I was quickly beaten, but because I was familiar with the previous games I adapted to its new battle-system.
Everything about this game has been improved, especially my favorite one, the music. Get the OST.