Played it twice over the span of 6 months. Although some puzzles were frustrating at first, on the second run, having already been used to the letter system, things went smoother. It was hard for me at first to grasp the 9-buttons with letters instead of numbers on them.
The sound design and art are amazingly done, and the atmosphere truly adds to the puzzles. I like how they are thematically fit for the game. Keeping things in black and white was also a great choice, as playing with greys is fun in the puzzles and it also kept things mysterious all throughout.