I give it a 6.7/10. The one line about humans just acting on their instinct without their self-control sums up what this manga is about. Reinforced by Subaru talking about how love doesn't need logic, etc a few times.
Would have a higher rating if it went through exactly what keeps them together. The best thing that'd explain their bond is the description of the story. Like the one on wikipedia.
It's strongpoint is that it's extremely direct and honest about it's purpose.