Separated brom the Mastiha Sour recipe by a little sweetness and a dash of Bolivar bitters - both are very good and refreshing but I liked this just a bit more. I think the bitters add more interesting and complementary flavor notes to the already delicious combo of mastiha and (sweetened) lemon juice.
Got bottle of Skinos Mastiha on my holidays on Crete. For this brand I recommend to avoid sugar syrup as the liquor is quite sweet itself. Otherwise great twist on Sour!