The Tartarus Key strikes a near perfect puzzle difficulty for me. I never immediately knew the solution. The puzzle would feel difficult at first. Then, after trying a few things, I would take a moment to think through all the clues in the room again and have my eureka moment where I solve it.
The atmosphere is the real star for me with this one though. It was very reminiscent of Resident Evil 1, finding yourself in a mysterious, creepy mansion with something sinister pulling the strings. Not to mention all the flavor text for the creepy paintings and other grotesque subject matter you can click on that adds so much to the setting and adds to your understanding of the protag's feelings and motivation/thought process.
My main disappointment was that the mansion was not as interconnected as I had hoped. When you enter a room, all you need for that room is in that room. So much so that you can't even access notes and items you gathered from rooms you are not currently in. I would have liked the mansion to feel more like one giant puzzle rather than a series of puzzles, personally.
I wholeheartedly recommend this to fans of this aesthetic and/or genre. It's also at a great price! But I don't think this will wow anyone who isn't already a fan of this stuff.