via Daily Motion, part 1:
DVD Talk highly recommends it, saying it's "not to be missed," and says, "It's a man vs. nature tale worthy of Werner Herzog, one with hubris, misplaced ambition, and even a touch of romance. Letter Never Sent is both human and elemental" and calls it "a wondrous thing to witness unfold."
Slant Magazine gives it 4 out of 5 stars. Rotten Tomatoes has no critics score, but the audience rating there is 83%.
In this film [director] Kalatozov represents something like the original source waters for one the cinema's greatest tangential histories - that of the plan sequence art film, beginning here and progressing to Tarkovsky, Jancso, Angelopoulos, Sokurov and Tarr. It's a style of cinematic experience that galvanizes your attention, as the world we see through the camera changes with movement and time, and we are free to wander around within the shots as if they're three-dimensional events. It's a shared realism in a myriad of ways a "normal" film, with all of its cutting and eye-direction, cannot touch, but the extreme sequences in this style also rope in historic, cultural, even existential thematic ideas, just by virtue of their length, complexity and scope. You can have a film tell you about man's relationship to the wilderness, or to God, or to totalitarian history. But then you can have a film hold you by the hand and take you on the tour instead. And then the experience is yours.