Incredible amounts of accurate detail and realism. If you like micromanagement, this is your destiny.
The Good : Attention to detail on an unparalleled level. A more intuitive interface, with a more lively map and more map modes to make situations your situation more obvious.
The Bad : The soundtrack is awful. Occasionally slow frame rates (I have a mid-range system). On map-sprites are still ugly, despite an upgrade from Hearts of Iron 2. Tutorial covers everything....except combat (combat is HOI3 is very similar to HOI 2, which is why I recommend the HOI2 tutorial for new comers). Also, the new HQ system is more of a hassle than something which adds to realism.Twice it has happened to me, all of my save game files randomly corrupted and I could no longer load them. This was VERY frustrating
Overall : Well worth your money if you have plenty of time, are a fan of strategy games (especially other paradox games), or like ww2 history and other games just don't cut it.