I think that despite a century of practice we still get confused with discussing books and films, and comparing them, and in this post I've pointed out of some of that confusion.
The first question might be what an adaptation is. You could perhaps say that all films are adaptations of the scripts they are based on, but let's for now concern ourselves only with films that are adaptations of books or other self-sufficient written sources (like comics, short stories and plays). One thing to remark upon is that adaptations are almost always only discussed as such when it is adaptations of either classic masterpieces, primarily novels of the 19th century, be it Jane Austen or Leo Tolstoy, or modern bestsellers. When did you last read an analysis of Rear Window (1954) as an adaptation of Cornell Woolrich's It Had to be Murder or Rashomon (1950) discussed as an adaptation of two stories by Ryunosuke Akutagawa?
Another thing is that when discussing a film that is an adaptation you need to differentiate between the film in itself and the film as an adaptation. It might be a great film but a bad adaptation, in the sense that perhaps most of the book has been removed and the sentiments and/or morals have been altered. Or it might be a good, faithful adaptation but a terrible film, because the acting is all wrong, the direction hysteric and the cinematography unintentionally ugly. Since the film is the film, and not the book, it seems to me that it is the quality of the film, not the quality of the adaptation, that should matter, unless for scholarly purposes.
How faithful an adaptation should be is an open question, it all depends. Being too faithful might ruin the film, even though it might work for a TV-series where there is more time available. It is a common mistake among filmmakers that they try to get as much from the novel as possible into the film. Maybe it would help if we stopped using the word "adaptation" and instead used the word "interpretation".
Even though adaptations are so very common, and popular, one of the oldest comments in film criticism, scholarly and popular, is "the book was better", sometimes generalised into "the book is always better than the film". Even more generalised is the argument "books are better than films". Two common reasons being that a) books allow more freedom for the reader to interpret and/or imagine what is happening on the pages than a film does and b) books have more depth and complexity. To me these arguments are often flawed. Obviously it is one thing to like a particular book better than the film made from it, there's nothing strange or flawed about that. But to take it further than that I think is unsatisfactory.
The argument that the book is always better than the film is flawed partly because it only takes into consideration adaptations of great books. But hundreds and hundreds of films are based on bad books, which doesn't stop those films from being good. And why couldn't a terrible book be transformed into a great film? Because the art form is by necessity deformed in same way?
Another flaw in the argument is that it compares two things that are not necessarily comparable. Apocalypse Now (1979) is based on Heart of Darkness, written by Joseph Conrad, but which one is best, the book or the film? Yes, you could on an individual level prefer one over the other, but that is not enough to make it into some general principle. Besides, it is two different art forms and should be judged on different grounds. This is why it is a mistake to say that books are better than films. It is a bit like saying that apples are better than oranges. Why? Because you think so? That is not a valid point. How do you compare a long take in deep focus in the film with the description on the page in the book? How is one better than the other?
When you see a film it is true that you often see everything (but not always, violence might sometimes be suggested or hidden, as are often monsters in horror films) whereas in a book you must make the images yourself. But on the other hand in a book the only thing you get is exactly what the writer has put there. There is nothing else. When you watch a film there are always a lot of things which are just there, even if nobody put them there especially for you. A street scene in a film will be filled with people, buildings, the clouds on the sky, birds flying and so on, and you can choose to look at that instead of the main characters arguing in the centre of the image. When you read the book you don't have the luxury to choose which words to read, you must read them all. Well, you could of course choose to ignore some pages, or the whole second half of the book if you find it boring, but that is not the same. I'm not saying one is better than the other, my point is only that you do not necessarily have more freedom when reading a book than when watching a film.
Maybe the bottom line is that we should not forget that they are two different art forms which cannot easily be compared. And that, here and everywhere, we shouldn't use our own personal preferences as if they were general truths about art.
I was partly inspired for this post by my friend Paisley Livingston's recent research on the subject of adaptations. See an example here: http://www.ingentaconnect.com/content/berghahn/proj/2010/00000004/00000002/art00007