In one sentence: A great idea that is more tiresome than enthralling in its execution.
This review will contain spoilers!
In many sentences:
The Man in the High Castle is initially extremely engaging because of the unique premise that Dick creates. Not only that, but he the various ramifications of Japan and Germany having won WWII are presented in a subtle way. The facts aren’t simply listed, but the world is developed and investigated through a few characters and their day-to-day lives.
Unfortunately, as the book progresses, the novelty of the premise begins to wear off, and the rather boring plot begins to make the book a slow, rather boring, affair. It's interesting, because Dick is so good at writing dense, interesting short stories, that I can’t help but wonder if this book may have been better served as a novella, or even a short story. That way the interesting idea can remain interesting throughout the work, rather than running out of steam halfway through.
Complaining about this having a slow plot, after just coming off of reading Titus Groan might seem odd, as that book is much more methodically paced. The difference, of course, is that the writing and language in Titus is so much more interesting, that even if less is happening in the story, the actual moment to moment of reading is more engaging because of the language and word-choice.
Another advantage this work would have had as a short story is in regards to the ending. From my reading, I take the ending to, ultimately, imply that the characters (in the author and Juliana) that they realize that the world they are living in is, in fact, a fabrication. This sort of "gotcha" ending would work much better in a short story, which can be read and consumed in a single sitting, and which doesn’t have the commitment of a novel. In novel form, however, this kind of ending is much more annoying, I as I find myself more committed to the book and expecting a more satisfying conclusion.
In the end, while the idea behind the plot is clever, and there are some interesting moments, I don’t recommend this book. Dick has better works, as do other science fiction (or speculative fiction) authors out there.