My mother gave me GWTW to read the summer after fourth grade. She wisely told me that GWTW is the kind of book that is fun to read and very emotional. It is the kind of book one should read interspersed between other more serious books ... and that way, the fun one is more fun and the serious ones have more meaning.Great literature? Certainly not ... more like an earlier version of popular culture ... but still well worth the reading.
It's not that well-written but the story line and the characters are unmatched by anything, short of the novels of Charles Dickens.
Who can forget red-headed Will the foreman, or the Slatterys, or Aunt Pittypat, or Dilsey, not to mention Mammy, Pork, Suellen --- how many minor characters are there? And they all fit and they are all believable.
And the writing is awful.
Your mom was right on the money.
GWTW's greatest value is the inadvertent truths within it. It's literary value is low. But I also would recommend it for the former.