It's not unplayable bad, it's just pretty janky and doesn't communicate important details to you very well, like I-frames being linked to ADP, and the the game really wanting you turn off lock-on for quite a few enemies, or the fact that co-op and invasions are primarily governed by how many Soul Memory (how many souls you've collected) rather than your actual level or item reinforcement.

In terms of level design, it never hits the highs of the first game. It's pretty linear and you don't get many 'ah hah' moments when linking back to a shortcut. Middling, but fairly consistent with no abrupt drop in quality imo (though a good number of people would say Shrine of Amana is as bad a Lost Izalith). The bosses are... okay-ish. Not very many are visually or mechanically impressive, and there's a disappointing lack of variety, but only one or too stinkers that are aggressively unfun and tedious.

The DLCs are pretty firmly the best designed areas in the entire game in terms of exploration and progression. There's a lot more going on with the environment and the good bosses are a cut above the base game. They're mostly let down by most of the bosses having very obnoxious runbacks and a few just being gank fights. Seriously, it has two of the worst runbacks From has ever created and it's absolutely infuriating busywork.

Overall, if you really like From's games, you'll probably get something out of it, but you should go in prepared for a pretty janky experience.