I recently started re-reading Dune. I did some googling and found this place.
I've read all the FH stuff and- shock horror- some of the KJA stuff. To be honest, I didn't find it bad, so much as unnecessary. Dune is still understandable without knowing the exact details of Paul's childhood- possibly better, even. I prefer to think of them as canon in a broad sense- what happened in them happened, but the details are fuzzy. This is, after all, a series which covers tens of thousands of years of history, much of that distorted by religious dogma and political machinations- not everything will be entirely consistent.
However, I won't be bothering with any future Dune books. They're just turning the series into a cash cow.
More of a Liberal Herbertarian.