Kick-Ass the movie is better than Kick-Ass the comic. Where the comic struggled with trying to decide whether it was realistic and down-to-Earth or crazy and over-the-top, the movie has no such confusion; it's the latter all the way.
A couple of the major plot twists from the comics are removed, and it's actually better without them, too; here you know from the beginning that Red Mist is Tony D'Amico's son luring Kick-Ass into a trap, and the story is therefore spared a twist that feels forced. Also, in the movie, as far as we know Big Daddy is exactly who he says he is, and there's only one allusion to the suitcase full of money.
Nic Cage plays Big Daddy far differently from the burly bruiser of the comic, and the character is better for it. He's a pencil-necked geek out of costume, and in costume he adopts a ridiculous Adam West cadence -- apparently this was Cage's idea, under the very reasonable point that anyone his age would have been first exposed to superheroes through the Batman TV show.
Hit Girl is the highlight, the thing that makes this movie different from anything you've seen before. She nails the contradiction between childlike innocence and sociopathic violence.
All in all, the movie is perfectly cast. Cage, Moretz, and Mintz-Plasse really shine. Johnson's narrative voiceovers are weak and forced, but he's just fine at delivering his dialogue.
The whole thing is great fun, improves on the source material, throws in some hilarious surprises for people who read the comic and think they know what's going to happen in the last fight, and is a good movie whether you liked, hated, or never read the comic.
Oh, and there's a line in there about "those old Ditko Spider-Mans you loaned me". Fuck yes; suck it, Stan Lee.