So, without further ado...
Three Movies That Get It Right
1) The Silence of the Lambs. One of my favorite movies of all time. Hannibal Lecter is also one of the best-presented characters in any movie, and Hopkins portrays him pretty much spot-on the way Thomas Harris wrote him. The movie also gets the feel of the whole novel down just right. A good book, and a great movie.
2) A Clockwork Orange. Though not entirely faithful to the novel--Malcolm McDowell is a far more likeable character in the movie than Alex in the book--this is another movie that captures the feel of the novel pretty much spot-on. And, really, the film's just such a big cultural icon anyway (for good reason).
3) The Princess Bride. Again, not entirely faithful to the book (though how could it be, with Goldman doing what's basically author's notes in the middle of a scene--remember kids, what works in a novel often doesn't work in fanfic), but a great all-around movie for most everyone. It's also obvious that Rob Reiner has a lot of respect for the material he's working with, and that comes through very well in the movie as a whole.
(Honorable Mention: High Fidelity)
Three Movies That...Well, I'm Not Sure
1) The Right Stuff. Don't get me wrong, it's a good movie in and of itself. However, there's not a perfect parallel between the book and the movie. There's lots of things in the book that didn't (or couldn't) make it into the movie itself (e.g., the whole culture of conflict between the Mercury program and the X-15 program); while cutting it might make for a more interesting film, it takes out a lot of the subtlety, background, whatever you want to call it. And again, even though this is a good movie, you'll still get more out of reading the book.
2) The Lord of the Rings. Again, this is another good series of movies. I've put this up here because they're not entirely faithful to the books. Now, granted, as kallah said a while ago, this infidelity is a good thing--all the damn elven songs and dwarven sagas get really old after a while. But there's a lot of stuff that got cut on the way to bringing the books to the screen. It makes the movie good, but it makes it problematic in terms of the question "Is this a good adaptation of the source material?" (It depends on what standards you're using to define "good adaptation." Anyway, it's a good movie, and that's all that counts, yeah?)
3) Clueless. Yeah, it's a silly teenage love comedy--but based on a Jane Austen novel (which were the silly teenage love comedies of the Regency. Which makes me rather wonder what the Regency equivalent of American Pie might have been, but I won't go there). Anyway, as "updated adaptations" go (you know, where you move the action from early 19th-century Devonshire to late 20th-century California), I think this is rather the best of the lot. Baz Luhrmann's version of Romeo & Juliet, I think, doesn't quite succeed so well (except as campy fun). I put it at this list because I know several people who would spit bile at the idea of making a non-faithful adaptation of any novel, and view films like this as travesty or worse. You can make up your own mind about it.
(Honorable Mention: Solaris)
Two Movies That Just Suck Compared To The Books
1) Contact. The book was an interesting intellectual exercise about how we might react to communicating with extraterrestrial life, and what it might "all mean" to both humans and aliens. The movie was an unsubtle preach-assed Chicken-Soup-esque circle jerk about how religion and science are really the same thing. (The opening scene--starts out in orbit around the earth, and you hear a huge deafening blast of current pop music; keeps pulling farther and farther back out of the solar system and then out of the galaxy, and the music gets older and quieter until there's nothing but static--is really good, which makes the rest of the movie's disappointments so much more frustrating.)
2) The Road to Wellville. I really liked the book, and I had such high hopes for the movie, since it starred so many actors I really like. But the movie took all of the interesting things about the book and turned them into one gigantic enema joke. And the ending was one of the worst cop-outs I've ever seen--bears absolutely no resemblance to the book in any way, shape, or form.
...I was going to do three on this list, just so I could keep symmetry with the other list, but I couldn't think of any other adaptation that really sucked. So feel free to write in with your own nominees for who should take the missing spot on this list.