Good stuff, Maynard is a catchphrase, popularized in the 1980’s, by a Malt-O-Meal commercial.
It is especially used in relation to food products, like it was in its original context.
The expression originates from a 1983 television advertisement of Malt-O-Meal where a kid is approached by his father, who is describing the cereal and finally exclaims “Good stuff, Maynard”.
Although there are some claims online, that the phrase actually comes from the television series The Many Loves of Dobie Gillis, which ran between 1959 and 1963, featuring a character Maynard G. Krebs.
Those supporting this claim state that the protagonist said it to Maynard at some point in the series, and it was referenced in the Malt-O-Meal commercial, however no one has cited any evidence to support this, and Malt-O-Meal even used the lack of evidence to copyright the phrase.