It was a bit hard at first, and understand some of the reviews. But you have to give it a chance. The combat system is "meh", and the game mainly focuses on economy and politics.
I wish the UI was a little better. Because I hate having the popup messages disturb my outline. Other than that, the game is gold!

Edit:
The game has changed a lot since I gave my first review. The UI has improved a lot, and there aren't as many annoying messages popping up in the outline. Influence (with sphere of influence DLC) is more important, and countries like Russia are more fun to play.
I do recommend buying the game with all the DLC:s as it improves the game experience by miles.

I own Victoria II, which was the second game I played by Paradox. It was the game that truly made me love this company for creating a game with high learning curve, loads of mechanics to explore which at the same time is ruthless and has a replay value by 1000% as the game changes over time as updates come.
The big difference between the two is that Victoria II:s economy system is totally broken, and probably unfixable even by mods. Other than that Vicky III has everything the pre-installment has to offer and moar!

This game is still a hard recommend by me.