I feel it will be hard to discuss this without liberal use of spoilers throughout, so turn away now if you don't want any.
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I saw the film before reading the novel, so I was not all prepared for the postmodern framing device around the main narrative (the film just dramatizes the main narrative itself straightforwardly, without any reference to the framing device). Many readers will probably find this framing device frustrating, because it completely calls into question the validity of the main narrative - the story of Bella Baxter as a sexually liberated, feminist, and radically left-wing Frankenstein's creature is marvelously creative and entertaining, but through the framing device, you can never be 100% sure that story is "real" (in-universe).

Gray bookends that story by presenting it as a found manuscript, and provides extensive commentary on it both before and after, presenting himself as merely the editor of a volume written by the "real" Archibald McCandless. Gray's in-universe version of himself has clearly concluded, with significant corroborating "evidence", that the found manuscript (i.e., Poor Things) is in fact a true autobiographical account of real events. However, in the interest of "fairness", he includes after the main narrative (by Archibald) a kind of rebuttal written by Bella (or Victoria McCandless, as she was then calling herself. This letter completely disowns and goes to great lengths to refute Poor Things. It is, however, so mean-spirited to Archibald that I simply cannot square it with the Bella Baxter I came to love in the main narrative, so I choose to agree with Gray's in-universe editor persona and accept Archibald's story as the truth. I would chalk up Victoria's letter to shock and grief, given that Archibald held it back from her until after his death, and neither he nor Godwin had ever told her the full truth of the story before. I interpret the letter as an elaborate psychological coping mechanism. The additional in-universe notes that follow dig up other "historical" writings by Victoria which sound nothing at all like that letter and very much like the Bella Baxter we know from the main narrative.

All of this is to say that I personally enjoy the framing device - it turns the entire thing into an enjoyable sort of mind-fuck, and leaves an ultimate verdict firmly in the hands of the reader. But if you prefer finality in your novels, you will probably hate this aspect.

The novel is actually more than just a novel - it's also an art piece, filled with delightful illustrations (including cheeky reprints of plates from the original old-school Gray's Anatomy: for example, a lovely diagram of a penis precedes the presentation of Duncan Wedderburn's letter), along with a surprisingly emotionally stirring and poignant reproduction of Bella's "original" handwritten response to her encounter with the poor of Alexandria. The novel is also strident and unapologetically left-wing and (dare I say) feminist in its outlook (this includes the main narrative and the framing device parts). It is in large measure a searing critique of both historical and modern society.

So, overall I would describe this as a wild, unusual reading experience which is certainly not for everyone. It is alternatively poignant, politically radical, absolutely hysterical (the whole Wedderburn letter bit had me in stitches), and as I said above the very pleasant sort of mind-fuck. In short, simply delightful.