Briefly: both brilliant and enjoyable. The setting is exceptionally creative, well written and internally consistent. There are novel mechanics which are both interesting and intuitive. The ergonomics are, for the most part, excellent: hinting is well calibrated; nearly every object comes into play eventually; boring actions often do themselves when they should; REVIEW reveals just enough; REVEAL ties up all the loose ends. I had to UNDO a few times, but never RESTART.
Hints lead you quickly to at least most of the essential volumes in the vast library, but it's well worth reading every word. The typing that takes does get tiresome; just pace yourself.
I hit only one hitch: that one big puzzle. I've solved many similar puzzles in other contexts; I think what stumped me was that this puzzle was so much more abstract-puzzly than the very organic rest of the game. The walkthrough linked to from IFDB gives, but does not explain the answer, so look elsewhere for gentler hints. It might have helped me just enough to say that (Spoiler - click to show)the song simultaneously masks and reveals the combination.
Though it didn't lessen my enjoyment, it occurs to me afterwards that while the game is extremely strange in places (the opening scene, the protagonist's identity, their equipment) it settles down into being mostly comprehensible, hence intuitive and playable. I would love to see the authors sustain that extreme strangeness through the entirety of some future game. Meanwhile, by all means enjoy and appreciate Sub Rosa!