I did not like this film the first time I saw it for a couple reasons : 1) I didn't expect it to be the kind of film it is, and I feel I missed the Mann's vibe because of that; 2) the storyline is more dense even than Heat with A LOT of those long Mann dialogue scenes that aren't easy to absorb as a not-fully-engaged viewer (which I was).
A few notes:
1) This is a film that you need to see more than once. If you pay adequate attention to the character development, dialogue, and events, you will be engaged with the action and drama that unfolds.
2) A lot of people have grievances with Colin Farrell's performance. It's kind of grown on me, actually, the more I've seen the movie. Just try not to get too caught up with it, because there are so many other quality elements in the film.