I preordered Civ 4 and got it last friday and have pretty much been playing it since. The game would be near perfect except for the fact that it is very unstable. And not unstable in just kicking you out to the desktop kinda way but either blue screen you or hard lock the computer kinda way and quite frequently at that. In my standard sized map game against 7 AI players the game froze up on my 3 times and then one last time after the first end game screen.
Otherwise though the game has been great and all the things I was leery of before the game was relesed was either handled well or not an issue due to the design of the game. Before the game was released I didn't like the idea of not having attack defense values rather one strength value but that seems to have been balanced greatly by giving each unit unique modifiers which gives most units some unique specialty that make them all important(rather than before where you'd just attack with the unit with the highest attack and defend with the unit with the highest defense). I was also initially wanting some better way of mass managing cities which didn't happen however the updates to the interface make it much easier to micromanage and changes to game play mechanics makes it more important to have well placed well developed cities over a large number of them.
Diplomacy works much much better with AIs now and you can have much more meaningful relations with them. Now there many modifiers to your relation with an AI(which are quantified and shown to). For instance you recieve a penalty if you're borders are pushing onto theirs from cultural clashes, or if you have a state religion which is different you also recieve a penalty(and a bonus if it is the same), trading with an enemy of theirs gets a pentaly, having peace for an extended time provides a bonus, etc. So if you build up a good relation with an AI they won't just randomly backstab you like in previous Civ games and will be more willing to make better trades. Of course it can also be dangerous. In my last game I was just freely trading with all the Civilizations. Two of them were intense rivals and both decided to decalre war with me since I was "trading with their worst enemy".
Religion is a cool new concept as well. Basically after a religion is founded it slowly creeps out(you can also help it out by sending missionaries to spread the religion). All cities which have your state religion recieve bonuses from you religious civic. Also an AI which has most of its cities a certain religion which generally convert to it. So it works wonders if you found a religion early on as it'll spread to your neighbors early on and make negotiations with them much easier when you share the same religion and it more than overcomes the penalty for having clashing borders. Though if a neighbor founds their own religion you'll likely have an enemy for the rest of the game since you recieve the biggest religion based relation penalty if you founded a religion which is different from a religion held and founded by another AI player.
Barbarians this time around are very brutal and seems to get better and better units(I saw them get up to swordsmen). They trash your infrastructure, slow down early expansion by a lot , scare off your automated workers, and will obliterate you if they have axemen before you have archers. Thankfully they aren't too bright and are seem to be force into attacking if they can even if they are at a huge disadvantage. So after many poor starts I finally figured out that the trick is to station a few scattered units in good defensive locations along your border to hold them back from trashing your infrastructure. In my first few games I made the mistake of just guarding my city with units which lets the barbarians just destroy all your improvements. And on that note the infrastructure built by workers is much more important this time around due to the fact that most the bonuses from special tiles you don't get unless you build some kind of structure on top of it with a worker(ie a pasture to egt the benefit of cows or sheep or a mine for copper or gold).
The AI this time around is excellent and fast(even with 18 players on a huge map the game ran through the AI turns pretty quickly). It is very aggresive with researching new technologies and I never managed to reach a point in which I was leading the tech race by a large margin(unless the player was in a bad position with no room to expand his empire past a small point) and was generally either tied or behind. The AI will also not trade tech unless it knows of atleast one other player with it(even if another player has it but they haven't met the AI will not trade the tech) or if the AI feels you are too advanced. Though I'm not sure if it is a bug or not but one of the computers who seemed to have 4 more techs than I did wouldn't trade because he felt I was more advanced :P.
Multiplayer is already in if you didn't know so you don't need to get any expansion to add in a half functional multiplayer mode like with Civ 3. I've started several multiplayer games and it seemed to work fine with no sync or latency problems. So if anyone else has it I'd love to try and set up a nice big game of it :). I just hope they get a patch out soon so I don't have to restart my computer a dozen times to finish a game :P.