The prelude books are OK and the sequels at least gave us some closure.
The butlerian jihad trilogy was awful though, it was there to give some necessary background information for the end of the series but when you've got characters with ancestral memories and characters that can see through time that probably could have been done in a much better way.
They (B&K) always claimed that their end of the series was based on the notes by Frank Herbert that they found at some point. They however, have never disclosed those notes, to at least give us an opportunity to verify those claims. I have a hard time accepting their revelation about Marty and Daniel. I also believe that their interpretation of the Butlerian Jihad deviates substantial from the idea of Frank Herbert. And that it is just the other way around, that they had to deviate from the notes by Frank Herbert (which it seems they only found after having written the prequels) to make them fit with those prequels.
A more likely reason is that most of them are really badly written. The books feel as though the writers wanted to make a quick cash grab using existing IP, but completely misunderstood what the exact appeal of the original works was.
The butlerian jihad trilogy was awful though, it was there to give some necessary background information for the end of the series but when you've got characters with ancestral memories and characters that can see through time that probably could have been done in a much better way.