It's addictive, it's entertaining, and it stays true to the civ formula. However, it feels too quick for my taste.
Civilization IV is addictive, well made, fun, and extremely vibrant. It retains the formula of the previous civilization and modifies it for the better. The sounds are excellent and give life to the sprawling 3d envrionment. Birds sing, cows moo, and you can hear the sounds of the waves lapping up on the shore.
I do have a problem with this game, though. A single game of civilization feels rushed in a way. You have a choice between Epic, Standard, and quick gameplay, but all of them feel inadequate in their own way.
Epic is meant to simulate a longer game, and it does this, but it does it by simply slowing down research and production, not content-wise. A game of Civlization III used to take days to complete. This was possibly one of my favorite features of Civilization III. You could feel and absorb each age as it went by. Civilization IV seems to just speed along. I hardly get the chance to appreciate being in a certain age, upgrading to a certain technology, or using a certain military unit. A simulation of guiding an entire civilization through time should take a while. Otherwise, it will feel incomplete.
Standard gameplay seems more epic than epic to me. In fact, I prefer it because you are given the chance to complete most buildings before the dawn of new ages, giving you more time to appreciate all the idiosyncracies of a particular age.
This really is my only problem. I'm sure others would appreciate that the game is shorter.