The thoroughness and exertion in Kant's work is astounding, and to see it develop into the post-critical period is near otherworldly. Reading the note variations really help to re-enforce the vast number of definitions and categories outlined in his Ontology and Cosmology [physica rationalis, psychologia rationalis]. It also serves as a nice refresher course for his Groundwork for the Metaphysics of Morals.
A word of warning, I walked into this having already read my fair share of Descartes, Spinoza, and Leibniz. I would have been lost if not acquainted with the basic understandings of substance and the varieties of epistemic and metaphysical modality.
Will certainly re-read. Looking forward to tackling the COPR.
To be honest/clear, I only read until the end of the Metaphysik Mongrovius, but I'm adding it now because I'll finish it at some point (have to return it to the library before then though).