I don't have time to do justice to Children of Men, but both Joseph Kugelmass at The Valve and Mark at K-Punk have written long, excellent posts on the film, and I would recommend you to them.
The film is, visually, extaordinary -- it led to one of those rare nights where I couldn't sleep, not because the baby was waking up every couple of hours (though there was that), but because I was haunted by the film's spectacular cinematography.
My one reservation with Children of Men comes from the slightly-too-heavy Christian flavor of the humanism in the film. The filmmakers definitely distance themselves from fundamentalist Christianity (the ‘repentance’ cult is seen as deluded), but it’s very hard not to read the Birth of a Child as enabling the Redemption of the Human Race in anything other than Christian terms.
Perhaps it’s possible to deemphasize this because the film brings in so many secular progressive/liberal themes-—the totalitarian overtones of the War on Terror and the Department of Homeland Security, the persecution of immigrants/minorities, and the potentially devastating consequences of pollution on both the environment and on human health.
But all that couldn’t help me from feeling a little confused during the scene where Kee and Theo were walking down the street and soldiers were making the sign of the cross—as if the film’s ideology was shifting under my feet, and I was being offered a Communion wafer when I had thought I was eating Junior Mints.