I liked it, though nowhere near the level of many.
First, it was one of those films that had to be seen close to when it was released. It doesn't have a lasting theme like some movies, and a first time viewer in 2013 wont like it as much.
Second, it's a dark comedy that tries to act like it isn't a dark comedy and that throws some people off. Many go into it, and watch the whole time, expecting a crime/vigilante drama that never really develops but at the same time the movie isn't intentional enough about being a dark comedy for many to switch to that type of viewing mode. If you expect a "serious" drama it's impossible to like this movie.
Boondock Saints is the crime/vigilante drama equivalent of "Starship Troopers" with better acting and a less obvious intent. Anyone watching Starship Troopers expecting a genuine sci-fi flick will hate it.