Notwithstanding other's concern this might be too sweet with the addition of sugar syrup, I followed recipe. While not too sweet, it was very cassis forward as would be expected, so if you aren't a big fan of blackcurrants, this one may not be for you.
Excellent cocktail, and as noted by Difford and others, cuttIng back on the DOM produces a very balanced drink by reducing the sweet component. In my view this drink demands the lemon oils / garnish, but curious to try Mike's suggestion of an orange.
Stumbled across this looking for a cocktail to use up half a grapefruit left from another cocktail (Cloister). This is an excellent cocktail which for me has terrific bracing acidity from the grapefruit balanced by the bitterness of the Campari and grapefruit. Notwithstanding other's comments, I'm not sure lemon or lime needed as I prefer the grapefruit to be front and centre which it does when made to spec.