I'd give it a B-/B overall.
I think the monster scenes and the tension during those scenes was REALLY good. Really played against my expectations of a monster movie. I also understand the political symbolism in the films larger events (Thanks to TVC for filling me in), but what I dont understand are the characters.
None of the interplay within the family was consistant (aside from them all acting distinguished mentally-challenged), and sometimes wasnt even coherent. At some point the dad is rambling about judging his kids mood on his farts. I mean I get it, thats asian humor. I just watched SARS WARS for gods sake. The difference, however, is that SARS WARS entire mood was humor, while the humor in The Host just clashed against the underlying tension from a film trying to be an honest to goodness monster flick.