I am a little ambivalent. On the one hand, the book hardly let me stop at some points, at others I felt incredibly bored. The description of endless halls and vestibules with the same names over and over again may have been interesting and fascinating the first time, but becomes very monotonous after a handful of pages.
The story is told through journal entries. However, our narrator is deliberately unreliable, which can be incredibly confusing for the reader here and there.
The looming twist also felt kind of disappointing in the end, as I thought the whole thing was grander in the whole scheme of things.
Nonetheless, Piranesi kept me entertained for long stretches, and if you want to dig deeper, you'll certainly find plenty of metaphors of great significance. Indeed, the book is very beautifully written!