You've probably seen it by now, but I'll post some of my thoughts anyway. First off, it's way better than Wanted, which I actually enjoyed, despite it straying WAY far away from the comic and it getting dissed by people. I haven't read Kick-Ass, but it felt very faithful to what I imagine the source material is like, and fans of the book have backed me up on that.
Some complaints - the movie felt like it didn't really know what it wanted to be/do. There were several distinct, (to me) conflicting tones and vibes. At times it was very realistic, portraying believable consequences to the characters' actions, etc., while at the same time there were many over-the-top, extremely unbelievable scenes and set-pieces. I kind of wish the film stuck with one style, or at least blended the several that were in the film more effectively.
I didn't like how they re-used John Murphy's music in this film, because I distinctly associate it with the movies it originally came from (28 Days Later and Sunshine). Mark Strong did a good job, but his character wasn't all that compelling of a villain. The "love story," if you want to call it that, was kind of weak.
There are probably some others, but those were the main things that bugged me. Overall, I really liked this movie. It's not quite as good as all the geek/fanboy hype that's been going around, but definitely worth your time imo. I had a lot of fun with it and when the movie clicked for me, it was was really great.
Lots of action/violence, memorable characters and moments and at times a subsersive take on the superhero mythos.
8/10
Oh, and two words - Hit Girl. She was the best part of this movie.