My premise is that there are several sci-fi/comic/fantasy artistic sacred cows out there that other artists have been reluctant to adapt or expand upon because of the anticipated backlash from "fanboys".
The first of those "sacred cows" that I'm going to discuss is the original Dune by Frank Herbert. It is one of the classic sci-fi novels and probably my favorite book of all time. I read that book and the others in the original series when I was in my teens. Loved 'em.

Starting in the late '90's, Frank Herbert's son Brian, along with co-author Kevin Anderson, expanded upon a lot of the back story that is mentioned when you read Dune. A lot of the information was firsthand from Brian's own conversations with his dad before he passed away and from his dad's notes. Since that time, they've written about 10 books, I believe.
While it is not the original Dune, and lacks the artistry of Frank Herbert, it's still quality sci-fi and serves the purpose of illuminating a lot of events and characters mentioned in Dune. These books chronicle events thousands of years before the events of Dune. I don't think it takes away from the original at all. I'm actually looking forward to rereading Dune with the added info.
With this generation of fanboys, Alan Moore's graphic novel, Watchmen, is held in even higher regard than Dune. A movie adaptation has been bouncing around for years with directors as diverse as Terry Gilliam, Darren Arronofsky and Paul Greengrass attached. Much in the same way that the LOTR movies were entertaining, Watchmen adds on to the world of the graphic novel without taking away from it. They are not meant as replacements but more as one fan's interpretation. And Zack Snyder is certainly a respectful, knowledgeable fan in the same way Peter Jackson was. While the Tolkien family viewed the LOTR movies more favorably than Alan Moore views Watchmen, that says more about Moore than it does about the movie. Moore does not even watch the adaptations of his novels and is openly hostile towards them.
I don't think that Snyder changed anything drastically that hurt the movie overall. Some of the flashbacks to the older super heroes were done in the intro. The comic within a comic about the sailor from a couple of hundred years ago is not included. How could it? It really wouldn't make any sense unless you had read the graphic novel. As it is, I'm sure that there are a lot of people that have seen the movie that don't understand everything because they haven't read it.
The casting in the movie is fine with the high points being Billy Crudup as Dr. Manhattan and Patrick Wilson as the Nite Owl II, but especially Jackie Earle Haley as Rorschach. He's fantastic. The low point being Malin Akerman as Silk Spectre II. She may be nice to look at, but she can't act.

The look of the movie is very faithful to the novel ... almost shot for shot. Snyder is the right guy to adapt graphic novels as his previous one, 300, did a great job with the look also.
The ending was changed a bit but there has always been some criticism that that was one of the weaknesses of the original story. I had no problem with the change.
So, overall, for fans of Dune, I recommend the Brian Herbert/Kevin Anderson collaborations. And for fans of the Watchman graphic novel, I do recommend the movie. Fans of all types need to lighten up a bit and not hold things quite so sacred. You're missing out on truly entertaining stuff. And just because you may enjoy an updating or reinterpretation of a story, doesn't mean the original is in any way diminished.
"Sacred cows make the best hamburger" -- Mark Twain







