Well, since I nominated it, let me get the ball rolling by posting my thoughts from when I re-read it a month or two ago.
I re-read The Man in the High Castle this weekend, having not a very strong memory of what it was about - indeed, one of the quotes on the back of my edition says "in Dick's fictional universe, truth is an idea cooked up to sell Perky Pat doll accessories," and I thought, Ah yes, I remember that stuff. But no: Perky Pat, I think, features in The Three Stigmata of Palmer Eldritch.
The Man in the High Castle, instead, is alternative history rather than strict science fiction. In a universe where the allies lost the second world war, the USA is divided like Berlin and controlled by the Germans (east coast) and the Japanese (west coast). There is a thinly-veiled rivalry between the two factions (jocular reference is made to Italy as "the musical-comedy new Rome," the victor that everyone forgets), which is about to boil to the surface in assassination attempts between the German and Japanese high command.
Meanwhile business persists, in particular the business of selling American Civil War and other cultural artefacts to Japanese collectors, who can't get enough of this shit. This leads to some of the most interesting discussion in the book, where we find out that some - or most - of the pieces are not real, and the characters wonder what it is that makes such 'collectables' valuable anyway. One tells us that it's 'historicity,' something that can never be seen in the object itself, but only in the eye of the beholder - which of course speaks to anyone who has ever goggled on eBay at the notion of a terribly-written, mass-market or even self-published book being sold for a hundred pounds to someone who will never read it.
This concern with what we value bleeds through the novel via the I Ching, the Chinese Book of Changes, which most of the characters use to guide their lives, in place of defunct monotheistic religions. We even learn that it has a connection with The Grasshopper Lies Heavy, the underground (but universally read, even by the Japanese) novel - written by the eponymous Man in the High Castle - which tells of an alternate present where the Allies won the second world war.
The meat of the story is in the journey, and the conclusion to the overall plot itself can seem a touch weak, though it has extraordinary implications. The point is that The Man in the High Castle keeps you thinking throughout, and has so much to say on every page, that you don't mind when the end seems one of the lesser parts.