It's been out for a while and I hear a ton of mix opinions on it.
The Patch:
-Mostly rolls back some changes that Conclave made that pissed people off; Paradox is constantly attempting to figure out how to slow down expansion without breaking the fun.
-Some fun little flavor things like new execution methods
-Some balance and GUI stuff
-Amazing optimization
-Disease map mode, but is mostly worthless without the paid DLC
-Adjust rules! Certain rules can be adjusted, changing things like Mongol invasion time, women being equal by default, de jure drift, stuff like that.
The Paid DLC:
-More intricate injury and disease system, honestly mostly just for funsies though it does seem to be a little more lethal. People like it for the most part because it gives them a sense of agency instead of just, "Oh, my chaste, unmarried brother has Great Pox. Huzzah.
-Prosperity, which is half of a dynamic system taking into account the fact that some places saw unusual plenty or lack during different periods. Good governance and investment improve prosperity, disease reduces it. You also get a "crown focus" which increase prosperity and, if you leave it on a prosperous county long enough, you can get new holdings, which is fun (especially when you put your capital on Iceland because no disease can reach it

)
-Flavor stuff: do angry things to prisoners, doom prophets, stuff like that; also a very controversial quest for eternal life, which can actually give you eternal life (though you can turn off fantasy content in the rules)
-Some "housekeeping" character interactions: force gavelkind (which seems mean), ask vassals to end specific wars instead of just Realm Peace, etc
-And, of course, the plague system. Replacing whatever there was before is a more complex plague system, where plagues are born somewhere, spread along borders and trade routes, infect people, destroy armies, and depopulate provinces (the opposite of "prosperity", and I think the only way to lower it).
-To counter plagues, a hospital system. New buildings built in the trade post/fort tab, they're enormous money sinks. Like, seriously. Aside from protecting you from disease (both making the disease less likely to reach the province, and reducing the economic impact if it does), there's also improvements that are mostly focused around tech.
I like it, personally. And I don't think it's anywhere near as divisive and Sunset Invasion. If anything, I think the argument might be that it's not quite enough (which is...maybe true), but the content is mostly fun and well-balanced (well, well-balanced-ish. For example, if you manage prosperity well you'll be making way too much money). The launch was also probably the smoothest in Paradox history ever.
All in all 7/10 would buy again.