Sin City (yes, it kicks ass), thus far, appears to be both a critical and commercial success (Rotten Tomatoes score: 79%, #1 box office opening week). It's also one of the most faithful screen adaptations of a comic book I've seen. It's flawed, to be sure, but its flaws are Sin City's flaws, not flaws unique to the film adaptation.
I haven't seen Constantine yet, and don't plan to until it hits the movie channels. However, from what I can gather from the reviews and descriptions I've read, they seem to have taken most of what was appealing about the comic books and thrown it out in favor of a much more generic Hollywood "fallen hero striving for redemption" concept. Critics seem to generally dislike this movie, and it hasn't exactly been a blockbuster success (Tomatometer: 44%, never higher than #2).
Now, I have no problem with someone making a movie about a guy who, as a result of an unsuccessful suicide attempt, can now see demons, and fights them in an attempt to buy his way into Heaven when he dies. As Hollywood "high concept" goes, I've heard worse. I don't have a problem with making a movie about Will Smith fighting robots, either. What I don't understand is why you would spend the money to acquire a license to the John Constantine character, or the I, Robot title, and then make this trite high-concept movie that changes so much about what made the books interesting and popular in the first place. People who aren't fans of the comic/book are not going to base their decision to see or not see this movie on the comic/book license: They don't care, one way or the other. People who are fans of the comic/book are going to be pissed off that you screwed it up. In other words, the only people whose decision will be affected, one way or the other, by the fact of the license are going to be less likely to go see the movie.
Now, there's enough of an "automatic" market for action-adventurey, special-effecty type movies that you may still be profitable, even after annoying the fans. But that "automatic" market doesn't care what you call it, or what the characters are named. And the sci-fi/comic book geeks would probably be a significant piece of that "automatic" market anyway, for a non-licensed movie, and to basically start out by changing things in such a way that you alienate part of your potential audience, and precisely that part that could have been the most vocal word-of-mouth advocates, doesn't seem to me to make much business sense, to say nothing of artistic sense. Unless geek word-of-mouth hurts a movie more than it helps it, but I find that hard to believe given the relative successes of, say, Lord of the Rings, Sin City, and the Spider-Man and X-Men movies. All of those are both financially successful and fairly faithful to the source material. Granted, faithfulness isn't enough by itself (Daredevil was pretty much faithful to the comics, too...), but it does seem like there is some correlation there, and it makes logical sense to me that there should be.