I used an overripe banana and both my wife and I really liked this cocktail. I think you are right that it is essential that the banana is overripe, otherwise you might have to compensate with creme de banane and/or sugar sirup. In my case I didn't search for an overripe banana for the cocktail, but instead searched for a cocktail to salvage an overripe banana that i was starting to get too mushy to eat.