...I still haven't made up my mind about Dune 2-3. Like Brian, I think they got worse, but I'm not positive about where it becomes critical. They're probably not necessary, but they seemed better and more integrated than 4-5, at least. (I've never read 6.)
It's interesting how the
Dune books are appreciated less as they go on, and many feel they get worse. I don't pretend to have any insights anyone else is missing (I'm sure I don't), but I have been re-reading these as you may know and I have a different take on them, which comes down simply to taste I'm sure.
I think the
style changes somewhat, from
Dune itself, to the sequels, but not the
quality. I also think the style change doesn't suit many readers who really enjoyed the immediacy, transparency and action in
Dune. With
Messiah onwards, Herbert started writing denser books, more idea-based, and with hints and suggestions replacing a transparency of events. I quite like the style, though, and there's a power and sense of grandeur in the sequels that expand his universe in very interesting ways. From my re-readings started late last year, I would say the sequel books are very good. Rather than drop off in quality as they progress, I would say they actually pick up.
Messiah is a little difficult, I feel. Coming straight after
Dune, the style change is slightly jarring and almost problematic, though there are events in the books that are very well captured, and it has strong merits.
Children is somewhat better (or more satisfying), feels broader in scope than
Messiah, and Herbert seems more in control of his world again. The story arc is satisfying, and it's well written.
God Emperor is actually better still. This book is really good - possibly the best sequel - and many of the ideas, and Herbert's imagery, is top notch. It's very thoughtful, and deeper than most SF.
Heretics and
Chapter-House are a pair story-wise, and I'm actually re-reading
Heretics now. I'm not that far through, so it would be premature to place it in order of quality among the sequels. That said, it's started well, and is very readable. It has a reputation for having more action than
God Emperor, so it shouldn't be too off-putting to those who are wondering whether to try it. I'm enjoying it anyway, and it's not a chore to read at all.