"Don't believe the hype" pretty much sums it up, poor AI ruins this game.
I've played each civ game since the first one on my old trusty Amiga 500 and even though its one of those games you have fun with and occasionally a little bit of frustrating with. The problem and I hate to beat a dead horse is the AI/Combat, its is pretty much broken. With Civ3 you could simply stack tons of units and more or less win with brute force however this does not work in Civ4, and because of that, you get a lot more ridiculous battle outcomes, Legion vs Modern Armor...you lose? Battleship vs Galleon...you lose? Seriously? Don't believe me, just google for yourself all the odd battle outcomes, the problem being the combat percentage is way off, having a 90%+ possibly win outcome means nothing. Attacking an enemy civilization requires you to have, and not joking here...you need to have at least 3 times the amount of units per city 0.0 because your going to lose a lot of units to stupid unbalanced outcomes, example ~ Enemy civilization has 10 cities and is defending each city with 3 units, that is 30 units. Meaning you will need to have, not joking at least 70-90 attacking units and even then, might not be enough! Chalk that up to unbalanced combat mechanics, cultural defense, tile defenses such as hills etc etc...meaning lesser units quite often win, and equal units...its best to defend and then counter attack, or just nuke the crap out of everything.
Also, I hate to say it but again, what F1tsy mentioned about cheating AI is true, such as enemy units simply appearing out of thin air are pretty much true, weakest civilizations randomly declaring war and then proceeding to have large groups of units sweep and capture some of your cities are all too common, lets not even talk about barbarian hordes attacking you when you start to get ahead, basically the game does everything it can to keep the playing field as level as possible, which in theory sounds like a good plan, but in practice it doesn't work too well.
Lets hope, the next Civilization game moves beyond these issues, or make another call to power game..