Crusader Kings II is a grand strategy game similar in scope to your typical Paradox game. Crusader Kings has one defining element that sets it apart from almost all other strategy games, and that is who you play as. In a typical grand strategy game, even other Paradox ones, you play as “England”, “Carthage”, etc. But in CK2, you play as a family dynasty. You might start out as the king of Ireland, but by the end of the game you could be on the other side of the world as an Indian Raja, all due to dynastic marriages, wars, political schemes, or a combination of the 3. This gives the game a great sense of freedom because so much can go right or wrong and change the outcome of the game entirely.
The complexity of Paradox games is well known and usually a blessing and a curse. CK2 suffers from a severe lack of accessibility due to the huge amount of intertwined mechanics that dictate the flow of the game. However, once you have deciphered the basics, the game fully utilizes all these mechanics in interesting and unique ways. There are in depth religious mechanics for most of the major religions in the game. You will have to deal with disease outbreaks all over the world. If you are an Eastern country you will often have to interact with the Chinese emperor and try to maintain good relations with the superpower. You have the ability to completely alter the course of history, or to try to follow in its footsteps.
One of the strongest points for CK2 is the community. Aside from a sizable selection of players who seem to enjoy crusading against Muslims a little too much, there are a lot of dedicated players who have taken up modding to help sustain the game. There are dozens of complete overhaul mods that change some combination of the map, time period, universe, and factions. Being able to play in a post-apocalyptic version of the United States is a great change of pace from the usual medieval era / gameplay. This adds even more replayability to a game that already has a lot of variety compared to the average grand strategy game.
As with all Paradox games, the biggest negative is usually the exorbitant price of all the DLCs. For CK2, this is no different. There are some notable DLCs commonly considered fantastic and necessary, but many DLCs only affect certain areas of the world, meaning that if you only play as Muslims, you probably won’t care about a DLC overhauling pagan religions or India’s government system. I have gotten a lot of enjoyment out of nearly every DLC but as always, your mileage may vary.
At a minimum, I would encourage people to give the base game a shot. After you feel like you’ve given it a fair shake, if the game is still too complicated to be fun for you, there’s no shame in passing on CK2. I believe most strategy fans will enjoy it on some level, but it is definitely not for everyone.