I really like the idea here, and I think the dialogue is, in general, implemented well. I enjoyed the plot.
The actual gameplay, outside of the mechanic, didn't offer much. Just press the correct key at the correct spot. Combat was spammy. Either I alternated between light and dark as enemies neared, or (for the boss) I was able to spam the appropriate attacks so quickly that I never saw multiple phases.
I only saw one ending, and I am sure there are others where the player is less balanced... but I never felt pressured to be unbalanced one way or the other.