Reign: Conflict of Nations is not a bad game. But it suffers from comparison with a great number of A-titles in the same class. If you enjoy epic historical battles and tactical thought, go for Medieval: Total War 2 (or any game of Total War series); if you enjoy historically truthful political simulators with high degree of complexity, go for Crusader Kings 2. If, however, you are not a fan of great battles and CK2 are too complicated for you... go for Knights of Honor. I'd really recommend Reign only to medieval eastern Europe enthusists as myself.
Reign: Conflict of Nations is an empire-building 4x strategy, depicting happenings in Eastern Europe between 1300 and 1700. Your ambition will be to unite one of the leading powers of the region (Poland, Lithuenia, Sweden or Russia) and become a hegemon. To this end, you'll need to build up a powerful economy, use diplomacy and spycraft, convert population to your religion, gather knowledge to make technological progress, and - of course - wage wars. Sounds promising, doesn't it? So... what's wrong with the game?
Firstly, you will be required to do ALL these activities at the same time and that's - even with the help of action pause - is extremely difficult for bigger empires.
Next, the battles are resulted by the computer, without giving you a chance to lead your troops or even influence the outcome. Moreover, the results are often unpleasantly random, so you'll find yourself kicking and screaming many times, after your highly superior army of knight has been wiped out by a rag-tag band of peasants.
Third, you control basically all aspects of the game from the very beginning and none of them (to my knowledge) grows any more important later in the game, so it basically feels all the same when you control a miniature princedom in the heart of Poland, and a huge empire of the late-game. You just do A LOT MORE of the same stuff. And this is a recipe for boredom.
Lastly, the game does not give you much choice of how you will rule your empire. In other game of this kind, you can choose to be a pieceful diplomat and trader, or a vicious tyrant, spying and assassining your enemies, or a steeled knight, conquering the world with fire and sword. Here, the game strangely pushes you into doing all - you can't win by the means of diplomacy, but you can't hold too many provinces with brute force (for unhappiness spawns infinite number of quite powerful rebels, who will finally defeat you anyway).
All and all, R:CoN is quite playable, enjoyable and challanging in the beginning, but largely boring, time-consuming and repetative later in the game. It might be good springboard for those who want to jump into Crusader Kings, but feel intimidated by their complexity. And it might entertain you if you're fan of Sienkiewicz and history of Polish and Russian empire. In nearly all other cases, don't waste your money here!