I'm sad to say (sad, since I do like some of London's other work) that it doesn't read very well and parts of it even reminded me of a left-wing version of Ayn Rand's fiction - which is a harsh judgement, I know - even if it is mercifully much shorter.
But it does have some redeeming qualities. The mixture of dystopia and utopia (which is hinted at in the 'footnotes' added to the main narrative) is intriguing and there are a few very harrowing and exciting passages (such as that recounting the doomed 'Chicago Commune').
I can't, in the end, improve on what Orwell said about it in 1940, in his essay 'Prophecies of Fascism', in which he compared it to H.G. Wells's The Sleeper Wakes (reprinted in The Collected Essays, Journalism and Letters, vol. 2, pp. 45-49):
As a book, The Iron Heel is hugely inferior. It is clumsily written, it shows no grasp of scientific possibilites, and the hero is the kind of human gramophone who is now disappering even from Socialist tracts. But because of his own streak of savagery London could grasp something that Wells apparently could not, and that is that hedonistic societies do not endure.
In this essay (which is what originally inspired me to read the book), Orwell denies what seems to have been a common opinion of his time, that The Iron Heel was an accurate forecast of fascism. He does, however, credit London with having insight into the difficulties of promoting a transition to socialism (something that Orwell - like London - advocated) and also into the psychology of fascism.
In an intellectual way London accepted the conclusions of Marxism, and he imagined that the 'contradictions' of capitalism, the unconsumable surplus and so forth, would persist even after the capitalist class had organized themselves into a single corporate body.
(Incidentally: that London 'accepted the conclusions of Marxism' is something I definitely do not recall learning when we read 'Call of the Wild' back in high school...)
But temperamentally he was very different from the majority of Marxists. With his love of violence and physical strength, his belief in 'natural aristocracy', his animal-worship and exaltation of the primitive, he had in him what one might fairly call a Fascist strain. This probably helped him to understand just how the possessing class would behave once they were seriously menaced.
In any case, the book's 'footnotes' (added by an 'editor' writing from a post-Oligarchy socialist future) are full of references to real people, events and texts from London's time. My favourite has to be the following one, about a city which is very close to my heart:
Chicago was the industrial inferno of the nineteenth century A.D. A curious anecdote has come down to us of John Burns, a great English labor leader and one time member of the British Cabinet. In Chicago, while on a visit to the United States, he was asked by a newspaper reporter for his opinion of that city. 'Chicago,' he answered, 'is a pocket edition of hell.' Some time later, as he was going aboard his steamer to sail to England, he was approached by another reporter, who wanted to know if he had changed his opinion of Chicago. 'Yes I have,' was his reply 'My present opinion is that hell is a pocket edition of Chicago.' (The Iron Heel, Penguin, 2006 , 221)
(If anyone can confirm the original source for this quote, I'd be grateful: the only thing I can find online is a reference to Ashley Montagu referring to it sometime later.)