Interesting for awhile
Darkest Hour is a challenge to master for newcomers, and you feel realised whenever you crack another piece of the gameplay mechanics, but ultimately it gets old fast and is riddled with small bugs and crashes.
So where do we start? The tutorials are very basic and don't tell you about alot of stuff you should really know before you start a real campaign. You will find yourself navigating the Paradox forums to try and figure out why it is that you can't build a factory in a certain territory (you need at least 40% infrastructure), or how can you get another tech team working simultaneously (you get one every 20 IC points).
You can select almost every nation you can think of from that period (certainly every nation I can think of), but whoever you choose, you and your allies have to compete against everyone else in the world. If however you choose a smaller country who perhaps didn't have much to say about the events unfolding during both World Wars, you might find yourself looking at the time passing by without much to do at all. Besides, by picking a small nation you start off with a lesser amount of Industrial Capacity (IC) than the countries you're supposed to beat and with less tech unlocked. That ultimately makes it almost impossible to surpass the big factions.
I've tried unsuccesfully to build up my own new faction in a WW starting with brazil and trying to form some sort of south american alliances, but even though I'm not at war with anyone else and my relations with Uruguay for example are the best possible, I still can't seem to get them to make an alliance. And if you want to be a major player in DH you have to join one since this will get you alot of free tech from your allies, not to mention trade agreements.
Since I've played this without playing almost any of the Hearts of Iron games, I wasn't really looking for the similarities, but just looking at a few screenshots you see how much alike these games are. The resources are the same, the economic management seems the same, the map is the same... Even the bugs make this seem like a mod that was in fast development. For example, my game was crashing everytime I got to 1916 or around, but at least the people at Paradox forums and tech support were nice enough to help with that. I still can't run this game at full resolution but it doesn't really matter since this is not a game that benefits a whole lot from that.
The battles take place on the same map you manage everything else and can take awhile. Diplomacy and Intelligence options are available to you, but unless you a very important country these won't be a crucial part of the game very often.
I still have to try out the multiplayer and will edit this when I do, but if it's just like the single player, I give the game 3 out of 5. It's still a good game, but aimed for fans of the genre and period rather than the mainstream.