If you care about the amount of gameplay you get for your money at all, this game probably rates higher than any other game. (Last I read, the average player had over 500 hours on this game.)
Crusader Kings II is a strategic game where you spend most of your time looking at a big map of Europe cut into little counties, but it is drastically different from a game like Medieval II: Total War. Whereas in those games, every nation had rigid boundaries and an entire nation was a distinct, unified entity, Crusader Kings II focuses upon the feudal system of governance, and especially its hereditary system of succession.
You are not a nation, you are an extended family through the ages. You may start as King of England, but lose the throne in a succession war, only to wind up inheriting the Kingdom of France, instead. It is a game of political machinations more than warfare.
You can't just declare war whenever you want to, you need to, by hook or by crook, manage to get a claim to that land you want to invade. Battles are relatively quick and hands-off (play that Total War if you want to manage the troops directly) but it's the space between wars that makes the game interesting. To get that claim, you probably need to find some way to get the princess of that land in your bed (or your son's bed), and it's this medieval bloodline plotting that make up the meat of the game.
For that matter, you don't necessarily need to declare war to gain land - just marry into the family that succeeds a throne, and make sure that, one way or the other, your chosen heir also manages to be the chosen heir of a target kingdom, and the kingdoms will be unified upon the death of your current character and the current king of the target land.
Likewise, just because you gain land doesn't mean you can keep it. You need to delegate land out, and succession laws can fragment your kingdom so much that you see hard-conquered lands slip out of your grasp by inheritence without your consent. EDIT: And since writing this, Conclave has come out to add complexities to keeping your nobles in line.
Worse, while whomever you give land will be quite grateful for the first 20 years of their rule or so, their children will have no gratitude for the gift of land your father gave them, and will likely be burning with ambitions you have to keep in check. Every few decades, your character ages and dies, and succession wars and civil wars are the primary way the game keeps your over-arching ambitions in check. Rather than letting a large empire mean that no external threat can challenge you, and no internal threats exist, the larger you become, the less vulnerable you are to external threats, but the more vulnerable you are to internal ones. You spend more and more time trying to find a way to bludgeon the rebellious nobles and religious minorities into line.
If you get tired of games of Total War, and don't bother completing them because, once you are the strongest power on the map, there's nothing to do but steamroll everything else that stands in your path, then CK2 is a great remedy to this. (You can still make a world-spanning super-empire, but it's much more interesting to try to keep it when the owners of that land still fight you when you plant your flag there than to just have them mutely pay taxes to you and have assumed loyalty to your cause.)
It's a game that's just hard and realistic enough to make the things you do actually feel like you're thumbing your nose at history. You can play an Irish earl, rise to High King of Ireland, and conquer England. Or play as the West Africans, and conquer the Muslims and convert them to your pagan faith. Being a norse viking that captures the pope and sacrifices him to Odin is always fun. (And nabs the Holy Smoke! achievement.) And if you do it with an unbroken line of Viking empresses, all the better.
Mods are also a high point: Play the Elder Kings mod, based upon the Elder Scrolls, and conquer Cyrodiil and beyond as an immortal or vampire, using magicks to summong hordes of demons when under threat. There's an A Game of Thrones mod, as well, and mods to turn the game into something like a space 4X game.
DLC still comes out for the game, as well. While more than a little annoying to be nickled and dimed, the trickle of new content keeps the game fresh and new achievements are meaningful challenges, requiring players to take up historic underdogs to rise to glory. Getting a full board of achievements is extraordinarily difficult, but enjoyable for the challenge they provide. Compare this to Activision, Ubisoft, or EA, where you buy iterations of the same game every year, and throwing down $10 for another year's worth of life in a game is well worth the price.