I found the film to be successful on a visceral level, but I was disappointed that the majority of the commentary was anti-government dystopia - the premise, not the setting, are of major importance here, but Cuaron seems to care more about the latter. Cuaron's films have always been focused on their locations, and I think his ability to place the viewer in a real, but imagined, place is noteworthy. Both Y Tu Mama Tambien and Prisoner of Azkaban share these great qualities. But Y Tu Mama Tambien was simultaneously about growing older AND rural Mexican poverty, while Children of Men is about the government oppressing people and pretty much nothing else.
Spoilers:
The newborn baby is trucked around the movie as a vibrant living image, one that impacts the world only in terms of visual shock - after this, the child is sent off to a mysterious organization that hopes to reproduce the reproduction, becoming yet another plot device with zero significance. Perhaps Cuaron didn't care about the distinctly religious story, or the human elements - instead he focuses far too much on political futures that become mere obstacles to the plot-messengers (Theo and Kee) as they work very hard to get the baby to her destination: a buoy? What is the significance of this amazing act? Why the long struggle for a baby? Theo's sacrifice and Kee's miraculous birth are treated as socially important, rather than spiritually insightful. This dystopian future seems poised to examine what makes us human, why humanity is so important to the Earth, and what significance WE have on our planet - instead, it is merely about a bunch of people screwing up, and one guy working very hard to get one baby to an organization we know nothing about.
But it sure looked DAMN awesome.