I have to agree with Bob Cook below that the fernet (and vinegar) get lost in the background. Maybe it's due to my grapefruit juice, but what I mostly got was lightly bitter grapefruit rum. I think next time I might drop the juice by 1/4 Oz and see where that takes me.
Wonderful cocktail. The hint of herbal and mint from the Fernet is perfect. The vinegar gives an interesting tartness, but is not overpowering. This is a cocktail that does better as it sits a bit and the ice melts a bit more.
I made this drink according to this recipe (even duplicating the garnish), and it's a nice rum punch. The fernet is totally in the background and doesn't overwhelm the drink, so this might be good for someone who is fernet shy. I've made a note to amp the fernet up to 7.5 ml and the apple cider vinegar up to 3.75 ml the next time I make it, but I can't guarantee that will be an improvement.
I didn't have Fernet Branca, so I thought "why not?" and substituted Campari for it, reducing the sugar syrup to the tiniest amount (to taste really). It was actually great!
Tried that recipe while dining, IMO grapefruit is not enough discernable in terms of tasting, maybe 2,5 oz of juice and 3/4 of each rums would be more satisfying, but what a lovely drink !