What is grapefruit juice good for?

Nutritional value +. grapefruit has nutritional value. However because of its metabolic effects it can interact with medications often increasing the potency of the medication. Therefore it's important that you discuss with your physician if there are certain medications that are you taking because it can now only increase the efficacy of the medication but can also increase the toxicity.
Yes. In moderation. However it may interact with certain medications like Statins hence should be avoided while on such medications.