It's quite good, but remember that it's an adaptation. Despite having to cut and change things the movie hit the main characters, plot points, and themes well. The human/spider/dog thing isn't in the books but is a perfectly fitting addition. As a fan of the books I saw it, knew where it came from, how it was made, and totally bought that the Baron would have one.

The only major quibble that I have with the film is a design decision. When it came to the costumes, sets, and even ships the film takes a minimalistic approach - probably to highlight how far in the future the story takes place. The books describe the noble houses living in much more opulence. A good example is the scene where the Baron is introduced. In the book his fat, ring-laden hand spins a globe of Arrakis made out of gemstones.

Other deviations from the book are necessary evils. The biggest one was how Lady Jessica acted when Paul was with the Reverend Mother. In the books all of her turmoil was internal and she was completely stoic. For the film it was necessary to have her show her feelings or do a Lynch style internal monologue.

If you liked the complete alien-ness of the people in the Dune movie you'll love the books. Be warned that they get stranger as the series goes on, but that's a good thing if you're receptive to Herbert's themes and ideas.