Hearts of Iron III is not an unmitigated success, but it is a success. The changes to general management are welcoming to newcomers, but also let veterans focus on what they enjoy most. The air war is much improved. The naval war is still underdeveloped, but this is a nut that few sims of the era have really been able to crack.
Most importantly, it just feels right. From the race to advance your radio and decryption tech to detect enemy strong points to the ability to open new fronts via diplomacy or daring amphibious assaults, Hearts of Iron III is the best representation of the military and foreign relations of the period yet made for the computer.