When I first bought CKII, I opened it up, stared at all the stuff on the screen, then immediately quit. I didn't touch it for another couple years. Granted, I also tried to play the Kingdom of Jerusalem right off the bat, which is recognized as one of the most difficult starts in the game, so that was probably a poor choice.
Once I did get into it, though, I really got into it, and I've put 240 hours into it, if Steam is to be believed. My most successful campaign was as Byzantium, where I took over Southeastern Europe, the Levant, the Maghreb, as well as France and northern Spain (taking territory from the Holy Roman Empire to make my territory contiguous is a right pain in the ass, let me tell you). Oh, and Dublin, somehow—I think it passed into my empire through inheritance.
(Ignore the demesne and vassal issues; this save game is from right after an inheritance (by a woman, no less, which rather exacerbates the problems.) Also heh, I forgot I married my nephew.)
Northern France (ie, the Kingdom of France) had briefly passed to Castile, but split away from it, and in the turmoil I managed to claim it on behalf of a relative, whom I later helped push de jure claims against Aquitaine and Brittany. Brittany itself was a weird case, because it had won Finland in a Crusade. England, as you can see, won Andalusia in a similar Crusade, which resulted in some really horrible border gore which is even worse than it appears on the map.
See the glowing bits? Kingdom of Andalusia. The non-glowing bits within 'England' are the Kingdom of England. No, London is not in England.
My dynasty is also on the throne of Scotland, and Catholicism has been more or less eclipsed by Orthodoxy.
Unfortunately, when the Conclave expansion released in February, it included myriad changes to the base game (i.e., the patch that affects people who don't even buy the expansion) that more or less made me abandon it. The changes to the alliance system I might've been able to live with (you previously got alliances from marriage; now you only get a non-aggression pact and have to form the alliance separately), but it also imported the shattered retreat mechanic from Europa Universalis IV, and, well, that mechanic is infuriating in CKII.
Shattered retreat, for those outside the loop, means that when an army is beaten in combat, it will run away, and cannot be engaged in combat again until it ends its retreat. What this mechanic is supposed to do is stop the process of 'ping-ponging,' whereby a strong army would beat a weak one, chase it to the next province over, beat it again, and repeat until the army is destroyed. Shattered retreat is meant to allow that army time to regroup or reinforce so it won't just be destroyed the first time it runs into a larger force. This works well in EUIV, where you can only move armies through your own territory, territory of nations who have given you military access, and your enemies' territory. Because shattered armies will normally attempt to retreat to friendly territory, once an army is beaten, you can logically deduce where it's likely to be headed, and send an army in its general direction. In CKII, however, military access is not a thing: you can move your armies anywhere you want, from Iceland to Ceylon, and the territories' owners can't stop you. Which means that a shattered army can be heading literally anywhere in the world. It's infuriating, and means you basically have to micromanage every retreating army, following them around province-to-province.
I couldn't deal with it, and I've sadly not played CKII since.
