I first read Dune in high school - so for me that would be more than 3 decades ago (but less than 4

) - I loved it! I immediately put it in the category of a classic (which of course it is). I've re-read it since then (and the sequels), probably a few times, but not recently.
As I remember, the miniseries was mostly faithful to the book - I found it a bit boring. I see that in the Wikipedia articles on the miniseries
and the movie, there are sections titled "Departures from the novel", which you can check out if you like:
http://en.wikipedia.org/wiki/Frank_Herbert%27s_Dune
http://en.wikipedia.org/wiki/Dune_%28film%29
The best that could be said about the movie (in terms of being faithful to the book) was that it was inspired by the book - I thought it was great! Even though it was totally different than the book, somehow I thought it successfully captured some of the
spirit of Dune - more so than the miniseries in fact.
Generally I don't like movies that go out of their way to be faithful to a book, particularly a classic. I've already read the book, know the characters and story lines, have my own images of all this ... why would I want to see all that again? Another factor is that seeing my own images replaced by someone else's is a bit annoying. Finally, for most books it is essentially impossible to be totally faithful to the book in movie form (for what I hope are obvious reasons).
I much prefer movies that are inspired by a book - a totally new take on various ideas and themes explored in the book. A good random example that comes to mind is Blade Runner - closely following Do Androids Dream of Electric Sheep would have resulted in a really lousy movie I think (instead of the truly amazing movie that by now has become part of the popular culture).
However if enough time has passed so that my images have faded to some degree, then I can enjoy a movie that closely follows the main story line of a book.