For the most part it is a pretty good expansion. I mean the amount of story and amount of new content is definitely expansion worthy. Anyone that says differently obviously hasn't played it. There are honestly a lot of just fake 1 star reviews because it is popular to shit on blizzard.
The campaign is better than I expected, it is a more return to the much darker horror aspects of diablo that I very much prefer. It isn't without issues though. Probably the biggest issue that it doesn't quite feel complete. Pretty much it feels like we only got 75% of the campaign at the moment. The length is fine and about what I expect out of a DLC it just felt unfinished. Likely they plan on finishing it in seasonal content but it just doesn't feel great.
Gameplay is probably the best it has been with decent variety and fun endgame activities. The problem is the gearing process is very unclear for new players and if you don't understand how the target farming uniques work it can be almost impossible to get your build off the ground. Trying to get your build uniques doing helltides or the pit is an exercise in uselessness. You pretty much have to go to the endgame bosses.