I just finished re-reading the series last week, including the last 2 books by his son. Those last 2 weren't completely awful, but they certainly didn't live up to the standards set by the original author. The only point that I was truly disappointed with the writing was when I read the following words: "male bene gesserit".

There were plenty of other inconsistencies as well, but as when watching any good movie, you need to suspend reality (in this case Frank Herbert's reality) and allow the story to play out.

I don't think I have the stomach to read the prequels, but I'm sure I'll come back and re-read the series again and again. Very few authors take the time to create such immersive universes.