Actually it is from a TV commercial in the late 60s for a hot cereal
called "Malt-O-Meal".

If I recall correctly, a father and son are at the breakfast table.
The father looks up from his paper and asks his son (Maynard) what
happened to his bowl of Malt-O-Meal. The boy replies that a bear ate
it. The father goes back to his paper while telling him about all the
vitamins and minerals and such and concludes with "Good stuff,
Maynard". There is then a knock on the door. The boy opens the door
and there is a bear holding a cereal bowl who says "More Malt-O-Meal
please."

In the midwest "Good stuff, Maynard" became a very popular catch
phrase as a result of the ad.

Please don't ask why I remember the commercial after nearly 40 years!

>Someone in here today used the catchphrase, "Good stuff, Maynard." My
>buddy has used that, too, and I never thought to ask him about its
>origin, but since he's too young to remember the old Dobie Gillis sitcom
>from something like 1959 and the beatnik character Maynard G. Krebs
>(played by Bob Denver), I figured that wasn't the source. For anyone
>else who doesn't know, here's the story on the catchphrase (somewhat
>embellished):
>
>http://everything2.com/index.pl?node_id=1516244