I tried it twice and could not understand at all what was supposedly so great about it. Granted, in both attempts I stopped pretty early into the game, but that's because I was hating every second of it.
For one, the story sucks. I don't mind games that want to tell their story in cryptic ways, but if you're gonna do it like that, don't treat me to a confusing lore dump at the opening cutscene, I'd rather just play the game and learn things at my own pace. For another, the graphics also suck. I don't need games to have highly advanced technology but Dark Souls is just so ugly, I was actually impressed it managed to look so lifeless because I didn't know it was possible. Everything was so gray and low-contrast, nothing stood out, nothing was interesting. Even the UI was hideous and opening the menu just made matters worse, what with its overwhelming collection of arbitrary RPG numbers scattered everywhere.
Then the gameplay... oof. It's one thing to make your game hard, it's another thing entirely to have a lot of trial-and-error bullshit where you get instakilled for not knowing better, and then that death makes you lose like 30 minutes of progress. Then you wanna get back to where you were quicker, end up dying again, and there go all your levels and now you're basically fucked and have to grind for an eternity. I like that the game actually punishes you for dying, but the way it goes about it is entirely wrong. People say "oh you only got instakilled because you didn't pay attention to this random detail!", well, yeah, I'm bound to not pay as much attention to details when your game is ugly as fuck and also has a confusing story that doesn't make me interested in the slightest. But more importantly, the issue is that I wasn't given a checkpoint close to this. If I had that checkpoint, got instakilled, and could quickly try again, that's a good way of teaching me that this game will throw random instakills at me if I'm not careful, and then I'd be inclined to be more careful from that point on. But by making me lose 30 minutes of progress, I'd rather just go play something else instead.
It was a wholly negative experience, really. That was several years ago, to be fair, maybe if I gave it another chance it'd finally click, but I think I'd rather play games I actually have fun with, there's several of those.