There were two things I knew coming out of the movie "Kick-Ass": that I loved it, and that it was going to be one of those movies that really divides people. Do not see it if ultra-violence pisses you off. Do not see this movie if little girls with foul mouths piss you off. I'd say don't see this movie if Nicholas Cage pisses you off, except he usually pisses me off, but he was perfect in this movie.
But if all these things merely make you uncomfortable, but you can see why they're entertaining, see this movie. It's perfect for you. It treads the line between entertaining the fuck out of you with the off-the-hook violence, and making you question how fucked up it is to enjoy these things.
Hand-wringing over Chloë Grace Moretz, who looks not a minute over 11 years old in this movie, cursing and slicing motherfuckers up has a tendency to miss the point of this movie. Are we supposed to root for this little girl and her deadly march through a drug kingpin's organization, or drop our jaws in horror at the idea of turning a child in to an unstoppable killing machine? The answer I got, and it seemed most of the audience got, was this: Both. (As he stood in the hallway as the movie let out, Marc heard many variations from fellow audience members of this: "That was awesome, but fucked up.") Most superhero movies cannot handle complexity in the slightest, but this movie deftly managed to convey that it was fucked up to make this little girl a killing machine, but now that she is what she is, it's hard not to root for her victory. This movie both relished the opportunity to engage in some ultraviolence, and portray vigilantism in a negative light.
Above all, the movie satirized the long-standing superheroes trope of minimizing the violence. Superheroes are supposed to be vigilantes, but in order to take the edge off the darker implications of that and to remain "family friendly", the stories have always kept the actual murdering to a minimum. And the character of Kick-Ass stands in for that---even though he doesn't have any super powers (in this universe, no one does), he decides to fight crime while carrying non-lethal weapons. And in contrast to Hit Girl (the real star of the show) and Big Daddy, he's an inept loser. But while effective, they are morally depraved individuals who enjoy killing people. In comic books, the superhero who loses a parental figure and goes on to fight crime is a romantic figure. In this movie, you're reminded that this is unrealistic, and that someone who reacts to a trauma by turning him or herself into a violent vigilante would be a fucked up person.
This is nothing new, of course. Interrogating the romantic tropes of superhero stories started with "Watchmen" and has, according to people who read a lot more comics than I ever have, gone on since then. But it's never translated well to the big screen. It's legitimate at this point to suggest that most people's knowledge of superheroes and all their tropes comes from the movies and not comic books, and so this lack puts the superhero movie watcher decades behind the comic book reader on this curve. The movie of "Watchmen" failed to really deliver---it had promise for the first half, and then completely lost it in the second. This movie manages to walk the line between entertaining as hell and disturbing throughout. Chloë Grace Moretz actually does a great job of being a combination of Buffy the Vampire Slayer and Rorschach from "Watchmen". If that sort of thing appeals to you, I think you'll love this movie.