Darkest Dungeon is just an slightly-above-average dungeon crawler with a lot of style instead of substance; but it does drip with that style (fantastic soundtrack, neat gothic storybook style, W.J.'s narration).
The game suffers heavily from repetitiveness and a lack of cohesive direction in its mechanics; it wants to be a dungeoncrawler, a hardcore tense combat strategy game, and carry horror elements all at once, but the devs both couldn't decide where to take it and lacked the balls to dedicate themselves to a dangerous, difficult, or hardcore experience. This is especially evident with the final DDQ.
It's a Risk Management game, everything from the initiative order to the dungeon layouts are going to be subject to uncertainty, but always with set rules and patterns (The Weald extends and crosses over itself, its a 1d8 for initiative each round, etc).
The game has a nasty reputation for demolishing people with attack accuracy and critical hits, but it's bundled in a warm blanket of confirmation bias– If you take the time to understand the game and prepare for the challenges, there's very little that will surprise or overwhelm you.
The game's balance has improved over time, as of Crimson Court's release it has finally started to come into its own after several long-standing issues and character flaws were fixed in that update's overhauls. It's not perfect, but it's not "four crusaders spamming Holy Lance and wiping every boss in 3 rounds tops" anymore.
If you're interested in a more cohesive, dangerous, and balanced version of the game, one of the people who worked on the game runs an overhaul mod called Pitch Black Dungeon, which is basically an expanded re-release of the game.
Crimson Court adds a pretty large chunk of content to the game (the big new thing is the megadungeons you can enter and leave while maintaining progress), and the new class is relatively fun. I haven't completed it, but if you enjoyed/pirated and liked the base game, it's worth buying.
tl;dr
its a risk management dungeon crawler that's stylish
pirate it, if you like it buy it and CC, play with PBD if youre there for the gameplay