Vindicated by Cable: According to a featurette on the Looney Tunes Golden Collection, Vol. 2, "What's Opera, Doc?" initially wasn't really any more recognized or popular than any of the other Looney Tunes shorts coming out during that time period. It wasn't until years later via TV reairings that people began to realize the sheer genius and masterpiece of a cartoon this is.
It was however given a prestigious premiere on the Warner Bros. lot, unlike other Warner shorts.
Apparently some symphony/opera radio stations have started to pick up on the cartoon and its music as well.