Personally, I can't help but feel a little let down. Sure, the rotational map and 3D is nice and all, but I just don't feel this has been as revolutionary for the Civ genre as it should have been. The new Civics system, while nice and easily customizable, just doesn't have enough options I feel, and it's obvious which civics are the ones to have and the ones to avoid, making pretty much every government in the world the same at the end of the game. Same goes for religion. As entertaining it is to have the Incan nation be the birthplace of Judaism, I don't feel it really adds a lot of spice to the game, being mainly just another thing you can do to generate culture for your cities.

I like the Great People thing though, and the interface is alright. The fact that workers now have more to do also pleases me. However, I keep on longing for my heavily modded version of Civ3, since there's just not enough new things in Civ4 to keep me interested for as long as Civ3 did.

What are your thoughts on it?

Well the best thing about Civ 4 is that it isn't as blatantly obvious that the AI is cheating.
Civ 3 was a bit like playing poker with your friends after they've shown you that they have 5 aces up their sleeves.
In Civ 4 I have yet to see a warrior destroy a single tank, something which happened regularly when I played Civ 3. I have also been able to launch surprise attacks on the AI on several occasions in Civ 4 which was impossible in Civ 3 because the AI knew the locations of all your units.

The other thing I like about Civ 4 is that it's gotten rid of the most annoying thing that was in Civ 3, the AI settler invasion. If there was a single piece of land surrounded by your territory somewhere on the map you used to get wave after wave of enemy settlers traipsing across your land just to get to it. You ask the AI to leave your territory and it would ignore you. You ask again and it would retreat back, but then... a few turns later, back they would come. The only way to stop them was to either place units on all the unclaimed squares or surround your borders with troops.

Thankfully in Civ 4 that's gone. The only way another civ can come onto your territory is if you sign an open borders pact or by committing war.