While yes, there are moments of action and intensity, it's really not what the movie's about at all. In fact, you spend most of the movie not even really sure what the "monsters" in Monsters look like. You know they're big, you know they're intimidating, and you know that they're dangerous enough that the entire northern half of Mexico is now quarantined as the "Infected Zone" and that travel through is pretty much forbidden. The United States has built a gigantic wall to keep the strange creatures out (one that would not look out of place keeping the Mongols out of China, only taller), while Mexico struggles with fences and regular air strikes whenever one becomes visible, as well as regularly spraying toxic chemicals into the jungles.
But again, it's not really a monster movie so much as it's a story about people trying to get from one side of a dangerous area to another, and seeing how everything changes when something comes along that doesn't fit with how we've constructed the world to be over the past thousand years. If blue whales suddenly grew legs and started walking around in cities, how would we react?
