I used to play CKI and wished for CKII. Bought it shortly after it came out but found it badly in need of more fleshing out and balancing. Set it down for a long time; four or five years I guess.
Thanks to getting interested in the history of Bohemia as a result of Kingdom Come Deliverance, I came back to CKII recently, reinstalled, bought ALL the DLC and content packs (only missing about half dozen of the music packs now, and I don't think I miss them).
My short and sweet review: base game CKII is good (maybe very good . . .). If you like it, then you will probably LOVE the full game, i.e., ALL the DLC and content packs. It is hard for me to judge the base game as it stands now because my most recent experience has been with the whole thing.
The whole game is astoundingly good. Years of play and replayability. You can explore almost any alternate history you want to explore using reasonably believable game mechanics and decently historical content. 9.4/10 for whole game. Probably about an 8.8/10 for base game, but I didn't really play it enough to say. ADDIT: the nice part about the base game being available for a 'normal' price is: it gives new comers an opportunity to check out the core game with a more limited time frame and less stuff and see if they like it.
Presently I've got about 200 hours with my current install, and probably had about 30 hours maybe 40 in the previous install (which only included about half the DLC and 25% of the content packs). Referring specifically to that most recent 200 hours of play: I've experienced nothing that I'd call a "bug" and certainly nothing game breaking. Good balance and good design, good 'emergent narrative' structure, quite good "game play" (meaning the observe, analyze, decide, and reap reward or punishment cycle and always with a risk of 'game over.'). That last part distinguishes it from my past experiences with EUIV, which (when I played it) lacked good over-arching integration of all the game mechanics and content to create an engaging game play cycle, and also had bad mid to late game balance (which . . . actually I cannot comment on for CKII as yet, still only up to 850 AD).
And then, once you exhaust all the possibilities of the whole "vanilla" game, there are lots of cool mods.
Just about the only downside to the whole game is the cost for the whole package: somewhere in the $100 ballpark on top of the base game, but of course there are sales if you are patient and watchful.
IMHO, if they wanted to push the sales tail of the game out indefinitely in six months or so, release a CKII Master Edition that includes pretty much EVERYTHING and retail it for about $60 or $65. They could probably even make up for the lower unit prices with more units sold even at a $50 price (I speculate wildly . . .).