I think film brings something that literature cannot bring, and its interesting you mention Dickens-his books are incredibly cinematic (hence the amount of interpretations) and go well with creating visual images.
When you go and watch a film adaptation, you invariably take with you the expectation that it will be as personal and powerful as that of the original, thats why adaptations of classic novels often fall flat. Very often the film-maker is left skirting the two extremes; does he remain entirely faithful to the narrative of the novel? or does he attempt to capture the 'essence' and dismiss the plot?
I agree that Dickens has had a rough ride by the movie industry, often they 'sweeten' his novels for a wider audience (the musical 'Oliver!' could be said to of done this) but when they get it right, its astounding...look at Lean's work on 'Great Expectations', especially the opening, which-in my opinion- betters that of Dickens' text.
I think the thing to bear in mind is that this ISN'T the same as the original it was based on, afterall its been adapted to screenplay first of all, then run through the film semic codes, then through the actors interpretation etc. By the time it reaches the audience it necessarily doesn't retain much of the original.
Film and literature are two completely seperate art-forms, I find that the novel engages you more personally, but suffers the disadvantage of personal interpretations-you can interpret a book in an entirely different way than your friend. Film is a medium that can engage and create a communal understanding, but lacks the personal nature. Obviously there is over lap between these two sides, but in crude general terms thats what I believe each offers.
Because of these differing qualities, I find that often one (very often film, given its popularity and status) side comes into sharp critism from the audience which is perhaps unfair, but understandable. It is this that needs to be addressed; are we looking at the adaptation as merely a 'visual novel'? an easy, passive way of understanding the printed word? or are we judging a film on its own merits, without saying things like 'wasn't as good as the book' just because it didn't fit in all the character exposition?