Having seen the movie yesterday and having 24 hours to mentally chew on it, I can't help but compare Kick-Ass to another similarly flawed superhero movie: Hancock.
Both are fun at points and INCREDIBLY flawed at others. Both are acted well above the level of the material the actors have to work with and both feel like they were working with two entirely different scripts at the same time and hoping they would somehow come together in editing. Which it never really quite does.
On one hand, you have someone on the writing staff who wants to legitimately turn Kick-Ass into a well-meaning, heroic figure, with stuff like his "three-on-one" speech and "No Power, No Responsibility" monologue.
On the other hand... you have what is very much Millar's influence with the whole "BLARGHABLARGHA! MASTERBATION JOKES! BLOOD! GUTS! SCHOOL GIRL LOLI WITH A GUN! FIRST PERSON SHOOTER SEQUENCE! BLAAAGAAAAH!"
So... yeah, Kick-Ass has its moments, but it's one of those movies that you should be prepared for a very uneven time.