I haven't read their _sequels_, but I have read their _prequels_ before reading the original dune and I found them decent. Not exceptional, but not bad at all. Slightly different from the original, though, which might be why they are off-putting to many people who have read Dune first.

(I am reading them in the order suggested here: http://www.amazon.com/Dune-Saga-Chronological-Order/lm/1COAD...)