"Not bad." That's one of the most frustrating opinions of all time.
"How is it?"
"So... what do you mean, exactly? Did it skip bad and just head on down to horrible? Does it mean it's good? If so, couldn't you have just said it was good? Or were you avoiding good on purpose? Is it just mediocre?"
I think you're the only person in the whole world who has problems with what "it's not bad" means. Are you like this with everything? "It's fine." "What does that mean? Fine like incredibly small grains? Is it like powdered sugar? If it's like powdered sugar, why didn't you just say that?"