First off, two of the three lead characters are awful. Blake Lively makes for good eye candy, but she just isn't a good actress. She's not bad in smaller supporting roles (like her part in The Town), but she simply cannot carry a lead. Unfortunately the entire film basically revolves around her (and she narrates), so her shortcomings are brought front and center. Every time her voice-over came on, I cringed. It really was not a good choice.
Taylor Kitsch is no better. Again... good eye candy, poor acting. Very poor acting. He just has no soul, and brings absolutely nothing to his character whatsoever. He should be in a brainless Fast & Furious movie playing opposite Vin Diesel, not an Oliver Stone drama.
The second major failure of this film is even more serious, and that is the story structure. We are never really given the opportunity to understand why the three lead characters (Chon, Ben and Ophelia) have such strong feelings for each other. We're told that they do, but never given any real reason to believe it. This is extremely important, because literally the entire premise of the film hangs on their relationship. If you're going to build a dramatic story around an unorthodox three- way relationship, you had better explain in more than one quick scene exactly how this relationship happened, otherwise the audience won't know why they should care about the characters. Especially when the actors portraying these characters aren't very good to begin with.
I know that I kept asking myself why these two guys share a girl, how they have absolutely zero jealousy, why they never once thought of double-crossing each other, and why either of them care so deeply for her -- to the point of being willing to risk their lives and commit horrible atrocities to save her. Where did all this love and loyalty come from? It was never adequately explained, and the entire movie suffers tremendously for it.
On a slightly more positive note, the veteran actors did a fine job. Benicio Del Toro was wonderful as a psychotic cartel underboss, John Travolta chewed the scenery to bits, and Salma Hayek was entirely believable in her role as well. Unfortunately, their competence only served to underscore the incompetence of the younger leads. It's telling that the best scene in the entire film was between Del Toro and Travolta, with none of the three lead actors anywhere to be found, and hinted at the promise this movie squandered.
A lot of reviews took issue with the violence portrayed in the film, but I didn't have a problem with that. You really can't make a movie about Mexican drug cartels without violence, so I didn't feel it was gratuitous. Unfortunately, however, it also didn't make the movie any more believable from a plot perspective.
Overall, I just don't think this was a very good film. I don't think that Stone felt entirely comfortable with what he was doing here, trying at times to be Tarantino but failing miserably. And likewise, I think that if this film had been in the hands of Tarantino or Robert Rodriguez, it likely would have come out much better, perhaps even great.