It's good, a serious challenge.
Some of the spots are in-sane-ly difficult to find. Which makes you happy when you DO find it, but with having only three "backup" runes you're pretty much guaranteed to fail at completing the game unless you re-start from the beginning. And since the hints don't show you the location that you were missing, you still don't learn from it.
Imo, the three backup runes should be changed to a "one hint" per level rather than a backup rune. That way you can learn and progress through the game by starting over at least.
Personally, I'd give up on this the way it is. I seriously don't feel like sitting cross eyed at my monitor any longer, hoping I'll notice three pixels being off. The blurry screen on the right make for the cake and just adds to the annoyance. Especially the darn blurry line that comes through so and then to distract you from your search.
And like most difference games, it'd be real nice if the screens were equalized when you find a spot. Rather than having a big red rune drawn on a spot, which kind of works distracting.
Also, the left and right screen are slightly (8 pixels) off. e.g. the left screen has 8 pixels more on the right side, while the right side has 8 pixels less on the left side. This will create the impression of something being different on the edges while in truth there's no difference there.