I loved this. Just so beautifully written and well-characterized in wonderfully thought-provoking ways. As Lash says, their characters are perfectly canon-compliant -- Millicent isn't nice and Greg isn't smart (at least not in conventional ways) -- but they are so much more nuanced and moving than in the books. I could read about this Greg and Millicent for hours. I love how the story depends on physicality: how they look, how the food tastes, how their emotions get translated into physical action (violence and nausea and hexes for Millicent; tears and cooking and beating up vegetables for Greg) -- of course that's just how things would work for non-verbal characters with deep feelings.
The backstories for Vince and for Greg's father are really effective and affecting.