"This berry is delicious!"
Translation:Tämä marja on herkullinen!
It's a different clause element, though, and rules about the usage of partitive tend to override rules about the usage of other cases. One of the things that require the usage of partitive is the object of an ongoing action. "Herkullista lörtsyä" is the object in that sentence and the verb indicates and ongoing action, whereas "herkullinen" in this sentence is a predicative.