I think I watched this movie under the ideal reviewing conditions. I, having read the comic, sat down and watched it with my wife, who hasn't read it.
First and foremost, this movie is an adaptation that take some liberties, so don't go into it expecting the panel by panel reproduction of the book that we got with Watchmen, 300 and Sin City. There are a lot of concessions made to the story to make it fit more in theme with a traditional Hollywood movie, like... I'm trying to dance around giving any spoilers away if you haven't seen the movie and/or read the book. It's really hard.
Okay, so, background. Our hero is just some schmo high school comic nerd, he lives in a world without any form of costumed heroes, and one day, he has the epiphany that why hasn't someone just put on a costume and gone out and fought crime? Is it that far beyond belief that someone would get tired of seeing bad things happen, and put on a costume and do something about it?
So, he buys a costume, and spends a lot of time posing in the mirror, climbing on rooftops, and coming up with a name for his costumed persona, finally settling on Kick Ass. The first time he actually tries to do something, he gets the crap beat out of him, and stabbed in the stomach. He ends up with a metal plate in his head, and pins and plates all throughout his body. But he doesn't quit (at least not in the movie) and eventually defends a guy getting brutalized by three thugs, you wouldn't say he does a particularly good job of fighting them even, he takes at least as good as he gives, but he manages to convince them to leave the guy alone. All along, someone captures the fight on a camera phone and uploads it to Youtube, causing him to be a new phenom.
Something about his fights just doesn't translate well to the big screen, it's lost a lot of the grit and realism that the book managed to capture. In the book he would frequently get pummeled to the point where you, me, or any other sane and normal person would give up, but he keeps getting back up and going back into the fray. You start to root for this, obviously insane, person, and you want him to persevere, and he does, but it's not always pretty to win.
He creates a Myspace page where people with problems can ask for his help. On one of his first runs, he goes to a crack den, and is way out of his depth, before he encounters two more masked vigilantes, Big Daddy, and his 10 year old daughter Hit Girl. Kick Ass is just some dick in a costume with a stick, these two are people that train all day long, and have access to all kinds of weaponry. Hit Girl is the one thing that I thought was nailed perfectly from comic to movie, she's tiny, deadly, foulmouthed, and yet somehow still a little girl. Nicolas Cage plays Big Daddy, and his costume is very much an homage to Batman, although he's a Batman who kills.
My wife had a fun time with the movie, and none of the things I'd mentioned bothered her in the slightest, as she didn't have anything to compare it to. The one criticism that we both had though, was that in the end SPOILER there's a jetpack, which just felt silly, stupid, and tacked on for no good reason END SPOILER With that one small caveat, I thought it was a really good action flick, and I'd recommend it to anyone.