The unique directional system was a nice alternative to difficult puzzles. It took time to get used to the "ups" and "downs" and "backs", and to get used to following the other characters. But it soon felt natural, and evoked a real sense of exploration.
Tension is effectively produced; my anticipation of the ending was pretty visceral. The game tells the player early on that something will go wrong, but the story is vague enough that every event seems a little more threatening.
From the (Spoiler - click to show)barking dog to a (Spoiler - click to show)breaking tree branch, everything seems to carry a little more chance of harm than it might otherwise. In fact, I honestly wasn't expecting (Spoiler - click to show)the game's open ending, despite the game being clearly labeled Part 1.