While I am enjoying the reading, I think it mostly valuable as highlighting a something-to-watch-for instead of establishing a this-is-incorrect. The general arguments which he presents (mostly elsewhere) against historicism are not very convincing. Instead, I feel that a historicist theory will require a very strong set of evidence which would be difficult to achieve. As far as I know, none has such evidence.
The key value is sociological, and limited; the text is a somewhat verbose supplement to Orwell's writing on the totalitarian mindset. For example, Orwell warns against the tendency to think in trends*:
It will be seen that at each point Burnham is predicting a continuation of the thing that is happening. Now the tendency to do this is not simply a bad habit, like inaccuracy or exaggeration, which one can correct by taking thought. It is a major mental disease, and its roots lie partly in cowardice and partly in the worship of power, which is not fully separable from cowardice.Popper says a similar thing in The Poverty of Historicism.
I find this with nearly all of the worthwhile insights and cautions. Avid Orwellians and Russellians will find them unsurprising. Perhaps as a hostile primer on Plato it has added merit...
*Note: I found via Google search a book which claims that Orwell was a historicist by his "attacking the antihistoricism of totalitarian regimes, that is, their notorious tendency to rewrite history according to the ideological requirements of the present political situation" (p.117). After reading this egregious abuse of terminology and noting the frequency of unnecessary citation, I think it safe to conclude that the author, Joseph Gabel, is a garden-variety French philosophe hack. Crude re-writing of history is not `antihistoricism'! But then, I should not be surprised to find stupid writing about Orwell on a Google search. Most of what is written about him is false and stupid.