The Sloe Gin Fizz is a classic and one that bartenders should put on their list, but there are a lot of variations being made out there. Some recipes use sour mix, some skip the gin and double the sloe gin, while others will fill with half soda and half lemon soda and some skip the sweetener all together. It's more of a matter of personal taste than anything else. I like this one because it cuts the sloe gin and the fresh lemon juice adds an element that sour mix can't touch here.
One last piece of advice... The quality of sloe gin varies greatly and that is why this recipe splits it with regular gin. If you get one of the great, crafted sloe gins that are appearing on the market, feel free to just use 2 ounces of that and skip the gin.