There are stages that nerds go through when a film adaptation of something they love is announced.
It begins with the glee. All it takes is rumors of a script being shopped around to send some people running to their favorite website to shout "They're making a ________ movie!!" (Inevitably they feel the need to use multiple exclamation points.) Rumors fly that some nerd favored director or actor is attached, IMDB entries are added, and fan sites are set up before anything is even known. Then 98% of these vanish without a trace since someone is always looking to make a movie with a built in fan base but that's a long, long way from actually making a movie.
Once a movie is actually being made the attitude changes. The realities of film making are settling in on the nerd dreams. The director is either a hack who has made some big but not particularly good movies or some nobody who's getting their first directing job having moved up from doing special effects. Whispers of things being changed are coming out of the set. And god forbid that the star doesn't look exactly like their favored depiction of the main character. There's grumbling now, fragmented statements of people declaring how much the film will suck and those demanding that regardless of how black the news that judgment should be withheld until the final result is released.
Then the big day arrives and there are inevitably two reactions. The best any adaptation can hope for from the fans of the original is a grudging acceptance. Some may say, "They changed some stuff but it isn't bad," but there's always a core that declares the changes as blasphemy of the highest order.
Personally I'm well aware of the complications involved in adapting something for another medium and make an effort to judge the final result on its own merits, but where's the fun in that? The question for any adaptation is "Blasphemy or faithful?"
Since comic book adaptations are popular these days I'm going to be looking at those more than any other. I mean who can complain that Transformers isn't faithful to a toy line that was built out of a random collection of licensed Japanese toy lines? (Please don't point them out to me; I'm sure they exist somewhere...)
Frank Miller's comic book about the Spartan's famous last stand at Thermopyle was turned into one of the most over quoted movies of 2007. I'm not going to use that line. You're welcome.
Pro: The art design looked almost identical to Frank Miller's original artwork at many points. The script used much of the same dialog (though a lot of that was borrowed from other sources too; I noticed Herodotus didn't get a credit).
Con: The subplot for the politics in Sparta isn't in the original and it added nothing to the movie. Spartans in the movie wore about twice as much clothes as they did in the comic (and I'll leave that mental image for you).
Result: Faithful. In fact it's about as faithful as you could possibly hope to get in a comic book adaptation.
House of the Dead
Uwe Boll, infamous as the worst living director, created this movie based on the Sega video games where the player shoots zombies.
Pro: There are zombies, they get shot. Was there anything else to do in an adaptation? Oh, and instead of actually making a movie they used in game footage for an extended period.
Con: Directed by Uwe Boll.
Result: Blasphemy, but one against god, nature, humanity, and film making rather than just against the game it was based on. With source material that thin it's hard to be anything other than faithful to it.
Bruce Timm compressed a full year's worth of four comic books regarding the death and return of Superman to one 80 minute movie.
Pro: The Doomsday half has a lot of Superman and Doomsday punching each other. Everyone thinks Superman dies, but he gets better. He comes back to life with a bad haircut and wearing a black suit. Superman has been enjoying a sexual relationship with Lois Lane without actually telling her anything about himself which makes him a bit of a dick.
Con: There's a... well it's not a homoerotic "subtext" if they come out and say it. Lex Luthor has a certain attraction for Superman in the movie (presumably something for Smallville fans). Everything after the death, which is about two thirds of the movie, is completely different. Superman's method for dealing with the villain at the end is extremely un-Superman like (dancing around spoilers there).
Result: Blasphemy, but well animated blasphemy.