Critics have never shown much agreement about the depth of Upton Sinclair's talent as a writer. Some feel that he was, at best, a weak storyteller, who hid his inability to create believable characters behind his sincere effort for political reform, while others have suggested that it was his political agenda that made it hard to see just how talented he really was. Most critics admit that he was fairly talented, though not exceptionally so, and almost all grant that he was scrupulously faithful to the details he wrote about. Sinclair's friend George Bernard Shaw, the Nobel Prize-winning playwright, suggested to people who asked what had happened in his lifetime that they should not look to newspapers but rather should read Upton Sinclair's novels. Bernard Dekle, who repeated the Shaw story in a 1969 article called "Upton Sinclair: The Power of a Courageous Pen," considered the author a "superb journalist": though that opinion says nothing of his ability as a novelist, Dekle goes on to quote Sir Arthur Conan Doyle (creator of Sherlock Holmes), calling Sinclair, "one of the greatest novelists in the world." Important literary critic Walter B. Rideout described the books of Upton Sinclair as comprising "one of the great information centers in American literature." Another much-admired critic, Granville Hicks, centered in on the quality that gave Sinclair the ability to be such a detailed and credible recorder of the world around him: "Sinclair," Hicks wrote, "has always had the ability to withdraw himself from the struggle and to write with an astonishing degree of objectivity."

But even those critics who praised his ability to capture events in words still acknowledge what Rideout referred to as his "artistic limitations." Rideout pointed out a discrepancy between Sinclair's fictional structure and his social message, explaining that they were separated from one another, instead of complimenting each other the way they should in a good work of art. Hicks, after marveling at his objectivity, recognized that Sinclair's writing, "if seldom downright bad … is not very distinguished." Even though he wrote his review during Sinclair's lifetime, he considered the author's works to be "historical fiction" because of the way they were meant to leave a record of the times. That, in Hicks' view, was the source of the problem: as he explained, "flatness of character is, I think, an inherent defect in the genre in which Sinclair is writing." In other words, even a really great writer would have trouble creating well-rounded characters if limited by the facts of history, and Sinclair was even more limited by his undistinguished talent.

One last area of contention comes from those who have disagreed about the level of objectivity in Sinclair's writing, and about how well it served the causes he supported. Few writers have openly criticized Sinclair for his support of the workers against people of privilege, and many have been willing to overlook the problems with his writing because they have considered him to be an overall positive influence. One critic who refused to give him any consideration for good intentions, however, was Van Wyck Brooks. Brooks rejected the claim that Sinclair's books recorded objective reality, pointing out that complete objectivity is impossible: "Mr. Sinclair, like the rest of us, has seen what he wanted to see and studied what he wanted to study." Since the world Sinclair presented to his reader could not be exactly the same as the real world, Brooks tried to describe what Sinclair's world was really like, characterizing it as one where "all the workers wear halos of pure golden sunlight and all the capitalists have horns and tails." Sinclair's supporters might still claim that it was his right to present reality as he saw it, but Brooks went even further, explaining that Sinclair's greatest failure was in not doing what he himself had set out to do: instead of showing workers to be proud and independent, Brooks claimed, Sinclair's over-simplifications made them look helpless and naive, like infants. The implication of Brooks' critique is that he personally supported the working class as much as Sinclair did, but that he did not think it did any good to overstate their problems or to understate their abilities to cope.