There has been some controversy with the film because of Chloe Moretz' depiction of Hit-Girl which you can read about below. The main thing to keep in mind before seeing this movie is that it is named Kick-Ass for God's sake. There is going to be alot of ass kicking involved, and it is for the most part a hard R and very lethal type of asskicking.
http://www.ugo.com/movies/kick-ass-controversy?cmpid=rss-movies-imdb
I already mentioned Chloe Moretz, above but I can't praise her role enough in the film. Hit-Girl may be the most messed up kid ever, and definitely someone I wouldn't want to meet in real life, but Moretz is again wickedly funny in the role, and more importantly she is the penultimate badass. Seriously this 13 year old girl could probably put the entire cast of the Expendables in the hospital, or more likely the morgue.
This kid is the real star of the show.
Nicholas Cage was pretty good too, which is high praise for him lately, of course his character is a total weirdo which helps make up for his current lack of acting skills.
In all I highly recommend this film but only if you like violence, superheroes, gunplay, morbid dark humor, as well as Superbad-esque dick jokes and likable dork humor.