Books 5 and 6 are a complete departure from the preceding novels, even more so than 4 was to 3. If you've read 2, 3 and 4, you know that's a good thing :).
The style is significantly different and they are set so far in the future that none of the characters and only echoes of the universe persist. Sadly, Chapterhouse: Dune was the last book Frank Herbert completed before his death leaving us with something of a cliffhanger. Despite the temptation, do not, ever, ever try the books written by his son and Kevin J Anderson. Despite their claims, I have a hard time believing anything about them came from mysteriously found "notes". Besides which they are just horribly written.