There is a marked change of quality - for better or worse, depending on the reader - in God-Emperor of Dune. There's also a large time gap between COD and GED.
I personally enjoyed GED, but if I recall Wilshire thinks that #4 is a drop in quality, though he likes #5 and #6. It pays to remember, I think, that the first three books were actually released as serials in SF mags, not read in novel form like we do. GED was the first in the saga that was written as a complete novel.
To each their own, of course.