First impression: compared to the movie, it blessedly avoids the black-and-white "East=good, West=bad" of Hollywood. The characters from both cultures are flawed and disappointing people in one way or another, as well as having their engaging qualities. Neither culture is idealized, too, which is equally a mercy. Both cultures, the one pagan and the other ostensibly Christian, are both quietly cast in relief by the ugly, emaciated figure that the samurai keeps seeing on the crucifixes of western lands. ...Oh, this is too much for me to really write about now: maybe I can think more clearly about this with a bit more time.
I'm just now struggling with a desire to move on to re-read Endo's Silence. I'm curiously filled with a kind of dread at the thought of going back to it, as though I were faced with a dangerous surgery: The Samurai could be hard to read, but Silence might be overwhelming. I had once thought of having my high school seniors read Silence and Erik Goldschmidt looked at me in wide-eyed shock and burst out, "Do you want to destroy them?!" That kind of book.
I guess that at the very least, that makes this a recommendation: if you want to read something powerful enough to shake you, then The Samurai or, even moreso, Silence will fit the bill.