Most comic-based movies completely strip the characters of what made them awesome in the first place, making people think that comics suck or some such garbage.
A comic-based movie that was actually better than its book?
"Blade"- they took a crappy character and made him awesome! Did you know that he
wasn't half vampire in the comics? He was basically a black Van Helsing that used 70's slang. Could you imagine if they had stayed "true" to the original work?
As good as their books?
"Sin City"- Almost word-for-word and panel-for-panel!
"Batman Begins"- It was recently voted as The Best comic adaption EVER!
"Hellboy"- Del Toro worked very closely with Mignola (the comic creator) to stay as true to the characters as he could!
Ones that disgraced the books they were based off of?
"Daredevil"- I always have to point out that the director's cut DID improve the story A WHOLE LOT!
"X-Men"- Too many changes from the comics to list here.
"Constantine"- Screwed up his origin and his name!
"Fantastic 4" &
"Fantastic 4: Rise of the Silver Surfer"- They SHOULDA been better!
Bunches and bunches of others.
As for straight films:
"Jaws" was better than the book for me because of the end. I've never been more disappointed in an ending as I was when I read the last few pages of such a wonderful story! (Yes. I know it was a 'Moby Dick' reference or homage or whatever you wanna call it, but DANG! It was just too exciting to have ended the way it did! I actually went back and reread the last coupla pages because I thought I had missed something.

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"I can't live the buttoned down life like all of you! I want it all: the terrifying lows, the dizzying highs, the creamy middles! Sure, I might offend a few of the blue-noses with my cocky stride and musky odor - oh, I'll never be the darling of the so-called 'City Fathers' who cluck their tongues, stroke their beards, and talk about what's to be done with this Monkey_Boy?!"