We enjoyed this but think that it benefits from a bit of simple syrup (or agave) just to round it out. We added just 2.5ml but maybe 5ml wouldn't be excessive. Our first thought was that it looks like a shy Margarita.
Grapefruit really dominates the flavors in this recipe, and the maraschino can't compensate for the bitterness. I may try cutting the grapefruit by 50%.