Russell Letson (chief reviewer for Locus Magazine for several decades) offered a substantial critique of Farmer's Riverworld series in 1977, just five years after the first book was published.
The article itself is too long to reprint in full (I've excerpted below) but in brief, he felt that the protagonists were heroic yet believable and that the books themselves were "sophisticated" and "grown up" and that they explored the limits of current sci-fi, all factors in Farmer's success as an author and specifically in relation to the Riverworld Series' commercial success.
One of the qualities that makes Farmer's work interesting—perhaps the
main factor—is his ability to break new ground in a field which, at
its worst (like all formula fiction), repeats standard patterns in
standard ways; this is most clearly seen in the motifs Farmer has
introduced or championed since the start of his career nearly
twenty-five years ago: alien sex and sex with aliens, pocket
universes, the Riverworld. I suggest that in addition to his
justly-recognized inventiveness in the realm of ideas, Farmer has also
pushed at the limits of the science fiction/fantasy conception of the
hero. Cawelti writes that the poles of hero design in most adventure
fiction are the superhero and the ordinary man, and that "more
sophisticated adults generally prefer the 'ordinary' hero figure who
is dominant in the fictions of those who are usually considered the
best writers of "grown-up" adventures.... Some of the most popular
writers of this type have managed to combine the superhero with a
certain degree of sophistication as in the James Bond adventures of
Ian Fleming" (Cawelti, p 40). Farmer has created heroes at both ends
of this spectrum, and it is probably his superheroes that show the
greatest degree of innovation in the use of the formula. The fictional
logic which results in the superhero, however, also affects other,
"lesser" characters, so that there is a constant tendency in Farmer's
work to go beyond the ritual and formulaic demands of the genre to
examine the nature of the hero and his relationship to ordinary men
and their world.
Farmer's work exhibits a fascination with the great hero, both the
fictional figure who in the past was the center and protagonist of
epic and romance and the historical man whose actions have
approximated those of imaginary heroes. Riverworld features the
explorer-adventurer Sir Richard Francis Burton and a number of
fictional and historical characters who qualify as heroic despite
their secondary roles: Lothar von Richtofen, Cyrano de Bergerac, Eric
Bloodaxe, Joe Miller. The historico-fictional world of the
Rivervalley, however, is less epic in its proportions than Farmer's
wholly fictional creations and continuations; the universes of the
World of Tiers, Wold Newton, and various non-cycle stories3 give us
the neo-Amerindian figures of Roger Two-Hawks and Kickaha, the
neo-Tarzans Ras Tyger, Grandrith, John Gribardsun, and the borrowed
figures of Doc Savage/Doc Caliban, Sherlock Holmes, and Phileas Fogg,
to name a few. These heroes coexist in Farmer's fiction with two other
classes of central character: the ordinary man who must act the hero
and the ordinary man who is transformed into a superhero.
Russell Letson - The Faces of a Thousand Heroes: Philip José Farmer