There's an interesting play between rum, vermouth and grapefruit which, with the right grapefruit, can be delicious. (Juice ripe pink or red grapefruit.)
Remade with Punt e Mes in place of the sweet vermouth and enjoyed it more, but I also feel it us quite a departure from the original. The interplay of grapefruit and PeM is delicious, but I still don't think this is quite it, I will have to keep experimenting