Though filled with many potholes and a steep learning curve, HOI2 sheer scope is enough to make up for it.
One can't talk about HOI and not mention the learning curve. It will take you days if not weeks to completely understand the options and tasks you must preform, hell, I still don't fully grasp the mechanics at play here. In addition to waging war, you must manage production, resources, diplomacy, government, economy, dissent, and a myriad of other chores. Don't be daunted by your first impressions of the game though, it just takes commitment to play through the tutorials and read the manual.
The scale of this game blows you away at first glance. you can play as over a hundred different nations, large or small, all with it's unique military, government, and international standing. Want to invade the soviet union as Japan? Maybe fight the battle for stalingrad coming from the east? No problem. How about fighting argentina as Brazil? lead a coalition of middle eastern peoples to fight their european oppressors? that's a possibility as well.
On another note, the graphics and sound aren't great. Though the musical score is dramatic and fun to listen to for awhile, it soon gets repetative and grating (besides "Japan Overture", possibly the very best song I've ever heard in a game. Look it up on YouTube). The absence of any sound affects whatsoever also can put people off. And while this shouldn't really make or break a game like this, the graphics are dated. Think civilization 2, if a half-step higher. In addition, some things in the game seem kind of quirky. For example, when playing as Japan, the royal navy destroyed my fleets every time they engaged, and Germany has forced the Soviet Union to the pacific in every game I've played. This is in stark contrast to the game's seemingly rigid historical context.
Bottom Line
Graphics: 2/10 but this really dosn't matter at all
Sound: 5/10 good soundtrack, lacking sound effects
Singleplayer: 8/10
Multiplayer: N/A They had multiplayer games available for awhile, but shut the network down because no one was using it
Replay Value: 10/10
In conclusion, HOI2 is not for the faint of heart nor the casual gamer. It simply is too hard to master. However, once you do, this is probably the best place in the world to rewrite history.