Yes, it's fantastic. And shockingly so! Though I've loved 3 of the 4 Lee Chang-dong features I've seen, I always hate the "tell the story in reverse" gambit (a la BETRAYAL or IRREVERSIBLE), and groaned when I saw it was being used here. But: alas, it's entirely necessary and perfectly done.
A film about the loss of human dignity, caused by our social and physical environment, and inextricably woven into our politics and history--it *has* to be told this way, and the "ending" (at the beginning) provides the way forward.
By tying it to his inner, spiritual state (rather than an outer, emotional state), it goes beyond the old "you never forget your first love" trope of something like Portrait of a Lady on Fire. Sun-im wasn't just his lover (indeed, the film withholds the "happy" scenes of their affair)--she was his link to innocence, of his life before the Fall.
Might be my favorite one yet.