I don't mind differences between films and the books or comics they are adapted from as long as the differences are appropriate to the medium and aren't just part of the process of shoehorning a thing from one medium into the other to make money even though its completely inappropriate and just not right for the medium.
My favourite film is actually an adaptation of a book, but the versions are centuries apart and come from quite different contexts, I have to say as well that I haven;t read the original but I know enough about it and the author to have an appreciation of it and how it informs the adaptation. That film is Salo, or the 120 days of Sodom by the Italian director Pier Paulo Pascolini. It manages to be an adaptation of a book that ought to be unfilmable, a blinding dissection of fascism and also a really harsh look at post industrial capitalist culture at the same time. Its every bit as horrific as the source material, bordering on hardcore porn, the only real difference between it and a hard BDSM porno is that its not really meant to be sexually arousing. Honestly, if you are going to watch it I'd recommend watching it a couple of times, you need to do that to get over the horror of what's actually in front of you before you can really start engaging with it. Because of the real fucked up nature of whats happening on screen it is easy to miss the depths of what Pascolini is saying. Its all about observation and the desecration of the human body, one of the themes was what Pascolini called the death of sexuality (his pessimistic take on the sexual revolution of the 1960s) I think the way that the thing prefigures and sort of critiques reality television shows that he was absolutely spot on.
To illustrate why its so good and how it works, there is a fairly infamous scene in it called The Shit Feast, that is pretty much what it sounds like. The shit feast is consumer culture in general, as well as a comment on fast food, and its the fucking Bush Tucker Trial on I'm a celebrity get me out of here and its every way that the system perverts your natural need to ingest. Everything that happens in it is significant and multifaceted in its meanings, within the overarching structure of meaning, i.e. that the Republic of Salo is actually Society.
Thats part of why i love it, that and the madness of the stories that surround it, the possible role of the film in Pascolinis death the year after it was released, the bannings and unbannings (James Ferman passed it uncut for viewing in the UK as a "fuck you" to the BBFC on his last day on the job). I think you could argue that its one of the most significant works of art by a Marxist working in any of the arts. Getting back to why its a good adaptation too, DeSade was an extremist, a revolutionary spirit of his age just as Pascolini was of the post modern age. Its a story with its roots in European royalist absolutism, set in a Fascist puppet state, but meant to be about contemporary society. The way it is both a critiques fascism by equating it with older versions of absolutism and then equates Fascism with Bourgeois liberal democracy. This in itself mirrors (deliberately I would say) the Marxist conception of society, political power and class struggle.
Its a brilliant film, and a reinvention of the source that brings it up to date and makes it understandable and relevant. Its the way you should so an adaptation of any book, be true to the substance of the work even if you have to rework the specifics.
I'll finish up with a short list of the films I haven't mentioned yet that I've enjoyed in spite or because of the original:
The Lord of the Rings (Jackson and Bashki versions)
Tank Girl (inferior to the original but still quite good)
The HBO TV adaptations of the Sookie Stackhouse novels and George RR Martins Game of Thrones.
Pretty much any version of Dracula but especially Murnau's Nosferatu
Ringu (I read an English translation of the novel and the film is so much better, the plot is just a lot better suited to the screen)
A Clockwork Orange (both film and book are amazing and done magnificently in a way suitable to its own medium, each perfect in their own way).
Interview With a Vampire
The Princess Bride
Can't think of too many more, and if I started listing all the adaptations that have profoundly disappointed or even annoyed me I'd be here all night.