Thought you guys might enjoy this.
Asspull: The prequels and sequels contain so many it's not funny. Most notable is how Omnius, the machine evermind, is defeated in Sandworms of Dune.
Recently, Brian Herbert and Kevin J Anderson said they used notes from Herbert found in a safety deposit box to write prequels and one sequel to the Dune series. They were not very well received, and most fans do not consider them canon. They're not bad (see 'Asspull', below), although they don't contain as much philosophy or ecology as the original novels, and their plots move much faster. FHM magazine once speculated that while they may have begun with notes from a deposit box, by the time of the last books they were down to a Post-it Frank left on the fridge saying "NOTE: Write more Dune books". Penny Arcade's assessment was a bit more blunt.
Note: in the article, the "not" in "they're not bad" has strikethrough
All in all, the authors and editors seem to have a pretty good understanding of Dune.