The reason you don't often hear the word boffo in every day language is that it is part of a sub-lexicon known as slanguage deriving from the Hollywood trade newspaper Variety. It's most common usage is in headlines of this particular journal and rarely is it found anywhere else--including in the articles of the journal. It's probably not a coincidence that it was picked up and used by Trader Joe's whose headquarters is not that far from Hollywood in California.