仮面の告白

In short I'd say it's a very self aware but also self indulgent book. As mishima himself admits:

ふつう、こういう考えは詩を書くのにはふさわしいが、小説を書くのには適しない。仮面の告白は、それを強引に、小説という形でやろうとしたのである

and even very early on I also had this sense that I'm reading something poetic, something like a drama or play. And I'm not saying this as a negative thing, just as an observation of the impression this book left on me. In a sense this book goes beyond the kind of thing you can rate with words like "good" or "bad" or a score like x/10 it's more nuanced than that.

Looking at it from a purely personal perspective, I can't relate to the protagonist and his seemingly unfounded sense of angst and dread at all (similarly to how I couldn't relate to the protagonist in 人間失格) , although to be fair it's not actually unfounded given the family he grew up in and his struggle with identity and his sexuality given the time he lived in, but I guess the way that he talks about his life such that it seems like there is this air of fate, destiny, drama around literally everything significant (and even minor insignificant things) that happend in his life. I realize that this is partially due to how the life of the protagonist is being viewed through the lense of 仮面の告白 so it might focus mostly on those kinds of themes and exaggerate some parts of his story for dramatic effect. I enjoyed the latter half of the book more where the protagonist describes events during his adulthood, compared to his childhood and teenage years, although those were important as a build up.

This is basically a fictional autobiography that at the same time might as well have been a real autobiography from Mishima, even if the events and characters are technically fictional. So he's showing his true self through all of these layers of meaning

TL:DR: the book is interesting, thought provoking, , daunting, difficult to digest, but ultimately a rewarding experience if you engage with it and try to fully digest it