Well, I bought two of them (the BUtlerian Jihad and Machine Crusade) because were having special prises, and just because of curiosity. I think they manage to sell the book because of Frank's name, and the really good image created by the original Dune.
This happens with alot of the books. For example Orson Scott Card: I was really impressed by the Ender's Game, and also by the Speaker for the Dead, but then the quality start to fade... and the Ender's saga ending was not my favorite. But overall, it was a nice saga.
The Shaddow saga success is just build ontop of Ender's saga. The first book is good, I would say as good as Ender's Game. But the next ones are not so good
. But because I have bought all the books in the series, I *have to* buy the latest one, just not to say that I stopped in the middle of the action. But if this last one (in Shadow's series) is boooooring, I WILL NOT buy any other Ender's Book!!!
And about SF: is somebody aware of some good SF writers (as good as Frank Herbert, Asimov, Clarke), 'cose I am a little bit out-to-date on this topic.
And back to fantasy: I think The Dark Tower can be a good choice, and not such a clasical heroes tale, but a more modern one. I have to continue with it (with the fourth book)...