I love Cash, and I don't think he's over rated at all.
He played and wrote a lil music just like everybody else did and does. It just so happens that his performance is timeless and crosses multiple generations. Not too many artists can say that.
He's rated right where he should be with that being an American icon.
Go listen to his Vietnam song or Man in Black. If it doesn't speak to the heart then you may be an ignorant tortured soul.
