My fortunes have been, from the beginning, an exemplification of the power that mutability may possess over the varied tenor of man's life
tl;dr version: More interesting as an artefact of early post-apocalyptic literature, and perhaps for the lightly hidden portraits of Shelley and Byron by someone who knew them very well. Hard going as a leisure read, but definitely interesting.
This is no doubt, one of the earliest of the post-apocalyptic novels (although the post-apocalyptic tradition itself is immeasurably older). From that point of view it's fascinating. The book is set in an early 2000's that looks remarkably like the 1800's, other than England is a republic, the king having abdicated. Otherwise, there is still a war going on in Greece, class is still the biggest societal divide, and really, the society portrayed is more of a portrait of what was going on when it was written than any guess at how society itself may have changed in the future.
Plotwise... well there's about 20 different plots going on at once here. It's very convoluted and involves many complicated love triangles and squares and possibly other polygons. Until, rather later in the book than I expected, tragedy strikes, as a vicious plague starts to kill everyone, everywhere. England, at first thought immune, quarantines itself, but eventually even that falls. It's terribly tragic, and awfully romantic (in the period sense, definitely not in the modern genre sense). As is typical of the time, and of Shelley's writing itself, it's quite dense. Here's the first paragraph:
I am the native of a sea-surrounded nook, a cloud-enshadowed land, which, when the surface of the globe, with its shoreless ocean and trackless continents, presents itself to my mind, appears only as an inconsiderable speck in the immense whole; and yet, when balanced in the scale of mental power, far outweighed countries of larger extent and more numerous population. So true it is, that man's mind alone was the creator of all that was good or great to man, and that Nature herself was only his first minister. England, seated far north in the turbid sea, now visits my dreams in the semblance of a vast and well-manned ship, which mastered the winds and rode proudly over the waves. In my boyish days she was the universe to me. When I stood on my native hills, and saw plain and mountain stretch out to the utmost limits of my vision, speckled by the dwellings of my countrymen, and subdued to fertility by their labours, the earth's very centre was fixed for me in that spot, and the rest of her orb was as a fable, to have forgotten which would have cost neither my imagination nor understanding an effort.
It doesn't really get any lighter from there either! I found I could only stand a chapter or two a day, before I had to go hunting for some lighter fare. It's also really really long.
You'll need either a classical education (which I don't have) or wikipedia on speed dial I think, to even make sense of a lot of the allusions. For instance the prologue is a tale of a journey to the sybilline oracles cave (and you are expected to know all about her already, which I didn't, much), and contains multiple quotes in several foreign languages. Personally I find that kind of thing fun if I'm in the mood for it, ymmv.
It's also fascinating reading if you're interested in Byron and Shelley. Mary was banned by her father-in-law from writing about Shelley in a real biography, so she wrote him into her novels instead, and here a main character (Adrian) is heavily modelled on him (albeit unwittingly, according to her own letters.) Meanwhile another major character, Lord Raymond, is apparently suspiciously like Byron, the original mad, bad and dangerous to know character. Raymond is certainly all three of those.
Readily available from Project Gutenberg among other places.