I found an interesting article here, with the title "13 book-to-film adaptations that the authors hated".
This part is on Alan Moore:
There have been several adaptations of Alan Moore’s comics, and there are likely to be a lot more. They vary in quality from the abysmal (League Of Extraordinary Gentlemen) to the acceptable (the “For The Man Who Has Everything” episode of the Justice League television show), but they have one thing in common: Moore has had nothing whatsoever to do with them. The man many consider to be the greatest living comics writer has always maintained that he wrote his stories specifically for the comics medium, and that any adaptation would be so different that he didn’t want to be a part of it; now, after four movie adaptations and a handful of lawsuits, he’s asked for his name to be taken off any property he doesn’t directly own, with the result that Watchmen—this year’s big-screen take on what may be the best superhero story in comics history—bore the name of only its artist, Dave Gibbons, in the credits. Given his extremely public, extremely negative, comments about the movie versions of V For Vendetta and League, it’s unlikely he’ll ever change his mind about Hollywood.
kinda long interview. Interesting fellow. Saw most of the films "based on his work" :) Interesting viewpoint.