Pohl offers in this work an extremely imaginative plot (though I don't know how original it is). But the story built around the plot is disappointing.
The plot entangles the fate, over a period of 10^40 years, of humans and a small family of sentient plasma-based entities. The latter, represented chiefly by one named Wan-To, possess not only high intelligence and power over physical matter and energy, but also very human traits such as jealousy, anxiety, paranoia, loneliness, and boredom. Wan-To's fear of aggression from clones he's created himself leads, inadvertently, to the acceleration to almost light speed of a small group of stars and a small human colony on a planet around one of the stars. As a result of the extreme relativistic time dilation - extremely slow aging - a small remnant of the human colony is able to survive 10^40 years, as experienced by the rest of the universe, including Wan-To.
At that very late era the "heat death" of the universe is almost complete, with all stars and even almost all protons having disintegrated. The residual human community has experienced a time lapse of only about 4000 years. And despite severe hardships along the way, they are enjoying somewhat of a renaissance, oblivious in their own private solar system to the near total demise of the universe around them. Wan-To, on the other hand, is apparently unable to tolerate existence in a pure void and is driven to consider seeking refuge in a surviving black hole (though even those are gradually evaporating).
This sort of scenario could plausibly be extrapolated into one in which the universe is able to sustain individuals and colonies of sentient inhabitants almost ad infinitum in spite of finite life spans. This would amount to circumventing the apparently inexorable progress of entropy. Unfortunately, Pohl didn't attempt (as far as we know) this extrapolation either in this book or possible sequels.
The plot, certainly, is imaginative and complex. It's also, except for the plasma creatures, based on reasonably good science (as known circa 1990). (Readers who find the science interesting shouldn't rely entirely on the accuracy of all details in the book.) There's a goodly amount of suspense. There are also surprises, although the ultimate final "surprise" was fairly predictable. Oddly, the main (human) character, Viktor Sorricaine, is clever and resourceful, yet he never seems to expect the final surprise until it happens.
The story's narrative leaves quite a bit to be desired. Of the various characters, only Victor is fleshed out to a decent extent - enough that the reader is able to feel somewhat sympathetic to his hardships and can feel a measure of elation with how well things turn out for him in the end. The other human characters are pretty much just stage props. There's little to admire in the plasma creatures, Wan-To and his clones. Apart from how they drive the plot, perhaps their best role is to provide a little comic relief.
Most of the narrative relates exhaustive details of the early triumphs and later tribulations of the human colonists on their adopted planet. Little of that detail is essential for the plot. It's just a fictional version of much of the devolution that's so typical of most real human societies into anarchy and cruelty. A later stage portrays a vestigial society dominated by several comical religious sects, whose divergent, risible belief systems aren't all that different from actual contemporary human religions.
The early ascendant stage of the human colony is notable for the prevalence of a "free love" ethic in which procreation is strongly encouraged for the sake of rapid population growth. While monogamous marriage exists, it's only one alternative. Unfortunately, the details recounted aren't salacious enough to be very interesting. Curiously, though the novel was first published in 1990, all the procreative activity is referred to antiquatedly as "making love", instead of simply "having sex".
Frederik Pohl was certainly one of the leading science fiction writers of the 20th century. His award-winning novels, especially the Heechee series, are much better examples of his best work than The World at the End of Time.