I think the film was an adequate adaptation of the book, but to me the book offered a much richer experience. I loved loved loved the book. Although i sobbed for about three hours after reading it, I didn't exactly find it depressing, but hopeful. When DH and I read it for the first time several years ago, he said it made him decide he was ready to be a Dad.
I think the ultimate message of the book/film is that amid all this darkness, there was still light--the intense love between a father and son. I sympathize with the mother (she has a much fuller presence in the film than the book) but really the father made the tougher decision to allow his son to live. I think a lot of people forget that there have been many times when civilization (on a large or small scale) has been at the brink of collapse. think of Europe in the Middle Ages during the Black Death, or even the Superdome after Hurricane Katrina. People do horrible things to each other when they are in direct competition for the means of basic survival. however, as the ending of the book/movie shows, community can still emerge from the ashes--it's up to "the good guys" to "carry the fire" of humanity and basic love for our fellow humans.
Also, I think the film--and many other "post-apocalyptic" works tap into an uncomfortable fear/knowledge that humans have the capacity to destroy their own world. We don't know in The Road what exactly has caused the destruction, but we know that a man-made or natural disaster could very easily plunge things into chaos. In fact, in the film, much of the footage of the landcape is from real places that have been destroyed or blighted in some way--there is footage of post-Katrina New Orleans, ruined sections of Pittsburgh and strip-mined portions of rural PA, and there's even a shot of the plume of smoke that billowed from the Twin Towers on 9/11. I live on the Gulf Coast and the threat of destruction of an entire body of water--and the livelihoods that depend on it--is very real to me, as the waves bring in black sludge that is like a reminder of our society's greed and overconsumption. I guess what I'm trying to say is that the film/book gives us an extreme version of very real things that are already happening. The author Flannery O'Connor was always asked why her stories were so grotesque and violent, and she basically said that she was dealing with an audience that was complacent and numbed by pop culture, so you have to shock your audience in order to get them to pay any attention-"for the hard of hearing you shout, and for the almost-blind you draw large and startling figures." I think this is what McCarthy has done.
Anyway, the book (followed by the film) gained a new significance for me after I became a parent. I would do anything to protect my child, and I think the collapse of society is a very real possibility. even without a total collapse, I fear for her--how will i be able to protect and guide her in a culture that is, in its civilized, technologically-savvy way, essentially violent and savage? How will I find communities for our family to be part of that will nurture her, when community is increasingly fragmented? I think these are questions that The Road engages, albeit in an extreme way.
Sorry for the lecture, but that's what you get from an English teacher