I'll get straight to the point, you pretty much only want this DLC for the new cool weapons and armor sets, and the boss fight at the end. The painted world of Ariamis is not very fun to explore, mostly because there's an overreliance on large groups of enemies, traps and poor visibility.
I would argue the final boss of this DLC is harder than anything in the base game and definitely the highlight of the expansion, but I don't want to spoil it. I'll only say that it's a long fight and I didn't find it unfair, just really hard.
However, I wouldn't recommend buying Ashes of Ariandel by itself, you should only get this as a part of the Deluxe Edition that comes with all the DLC or the season pass.