I enjoyed Kingsman: The Secret Service even though it is not a great movie by any stretch of the imagination. It pays homage to the old spy movies, particularly the older, campier, James Bond movies in their use of gadgets and plot points. I believe it's based on a graphic novel, and like most movies based on graphic novels, they tend to have flaws, mostly by feeling compelled to adhere to most, if not all, of the graphic novel plot points and/or characters.
On the plus side, this movie has enthusiasm for it's subject and takes great joy in showcasing fight scenes, gadgets and even the production design. Each is top-notch and the choreography/stunt work in the fight scenes are often times excellent. Some of it has been seen before, perhaps even 'borrowing' the style used in Robert Downey Jr.'s version of Sherlock Holmes. But it still works here, except when it gets overly gratuitous. Good, solid filmmaking and entertaining but probably not for those who are a tad squeamish.
I agree Kingsman was overly gratuitous.
Interesting comment as you seem to be saying most graphic novels are flawed.
Adhering to most of the major plot points and characters seems like that would be a good thing and no more of a tendency than when converting a regular novel into a movie, no?
I've read more novels and only a couple graphic novels so it's hard for me to compare and generalize. The GN that stands out in my limited exposure is
Watchmen. It has the kind of depth usually not seen in a comic, but it's a meandering, dark and depressing story. I'd call it flawed, not something I'd want to read again, but I'm still impressed by it because it approaches a novel in character complexity.
My impression is that any story based on a novel or graphic novel is judged on the merits of its narrative, coherence, excitement, interest, etc. And if something like Watchmen is considered brilliant, the challenge in making a movie based on it that adequate represents it while compressing the story down to 2-3 hours (or these days make 3 movies out of it

) While there are situations where the movie can improve on the book, most readers of a novel who love it, want loyalty, want the movie to remain true to the book, not turn cowards into heroes, not jumble the narrative, exchange characters, and/or put major characters into situations they never faced in the book (for example Game of Thrones or The Hobbit). To the movie makers, if you don't like the story, write your own book or give your movie a new name.
