I made this but substituted Leopold Bros. Apertivo for the Campari, which increased the bitter level somewhat (as LBA is quite a bit more bitter and less sweet than Campari, which I'm not particularly a fan of). However, I like bitter elements in food and drink, and this combo, with the sweetness of the sugar syrup, balanced out perfectly! It's a surprisingly multi-layered taste experience, with the bitter, sweet, and sour swirling around each other to create a full-bodied mouth explosion.
I made with a gin aged in bourbon barrels. It really made for an interesting variation. The Smokey bourbon flavor to the gin complimented the bitters and grapefruit wonderfully. Loved it.
The Campari and grapefruit enhance each other so well. I would say that depending on the sweetness of the grapefruit, the sugar syrup is hardly needed. Lovely drink!