Beautiful materials, really innovative mechanics for invoking that "prestige supernatural drama" feel. Really love the system of players being given tools to construct the mystery themselves, as I feel most mystery games just degenerate into "how well can players read the GM's mind?" Usually I'm very against players keeping character details secret out-of-character as it makes it harder to tease them out in play, but the Janus Mask mechanizes it effectively and ensures "dark, tragic backstories" actually get revealed in the course of the game.
The playbooks wear the game's Penny Dreadful inspiration on their sleeves and you'll definitely get a kick out of them if you're a fan of the show. Each are very unique with signature moves that have larger ramifications on the campaign, such as introducing new Threats or just giving the player a B-plot to pursue.
The Mastermind sheet is a very cool way of structuring the campaign, I would have loved to see two or three additional options but what's there gives you a good example to work off of if you do want to make your own.
Haven't had a chance to run it yet, it's got me fired up. I'm interested to see how players who aren't necessarily familiar with Victorian horror approach it, and how well the book's very evocative tone manifests in play.
[5 of 5 Stars!]