I've had this game for a little over a year now, and all I can say is: wow. The game may have a seemingly very steep learning curve, but once you play a few games, you'll easily get the hang of it. The game has very realistic researchable technologies (a la Me-109E German fighter) and of course, every country has their own unique unit types and weapons (smaller/minor countries will usually use a mix of Axis, Allied and Comintern techs).
The number of countries playable is also astounding. At the very least 50 in the earliest 1936 scenario. And the endless possibilties, don't forget! I once took over all of Scandinavia as Sweden and by 1941 had joined the Axis and fought very successfully against Russia in the north. And I once conquered all of north america as the United States and went to war with the whole world, with a truly unbeatable navy, army, and air force :D. For those that love Grand Strategy Games, the interwar and WWII era, and "What-Ifs" and ahistorical scenarios, this game is for you, all neatly integrated on a single disc.
And don't forget that there ARE expansions. The game on it's own is very fun and will sustain your gaming nerve for a very long time, no doubt, but expansions always add to the possibilties. I myself don't own any HOI2 expansions, yet I still play the game to this day.