Critics hated them, audiences loved them. And if you know about the home town, Flint, Michigan you would understand the passion in the music. Yes, the same Flint Michigan that Michael Moore wrote about.
What more can I say. As a grad student (OK I am just off the boat) my friends (still friends) pretty much advised Grand Funk was not the in thing. Apparently, Grand Funk didnt do cool guitar chords. I have vinyl of Grand Funk, and still love the music.