It's not bad.
The post-apocalyptic scenario doesn't bear much scrutiny (I scratched my head for a moment at the idea that something could kill all plant and animal life and yet leave humanity miraculously intact, but then opted to roll with it) and it doesn't seem to want to treat cannibalism as anything other than an eerie, unacceptable evil.
I don't know, given that nothing is alive any more and the only food supply is a finite amount of canned goods, it'd seem to me that cannibalism is something that would have to become a norm for the species to survive. Any functioning society would need some sort of system in deciding who to eat and why, and ration the supplies. Maybe different groups of humans would go to war to increase their food supply. When Viggo tells his boy that under no means will they eat human flesh, the back of my mind was thinking: Well, what if he's already dead, for one thing? They take supplies from people who are already dead, it's just a minor variation.
But no, in this film those who have resorted to cannibalism are those who have basically surrendered their humanity. Now I'm not actually complaining about that, that makes sense, and in a sense the society I suggested above would also lose its humanity - I guess maybe the inevitability of this is the film's fear, although the mythic way the Earth is dying implies that humanity may be slowly dying too.
I wasn't too happy with the treatment of the mother, but I guess that reflected the film's need to be poetic rather than rational
And the ending was, well.
But look, The Road isn't about cannibalistic society or the logic of how the world ended or any of that stuff. It's about a man and his boy travelling through the ruined world, struggling with the loneliness and the bitterness and the lack of proper food and the fear of being eaten. his son keeps him alive and is his justification for everything he does, to keep the fire and so on. In this the film basically excells and is a watchable and very nicely shot film, with an excellent soundtrack and some pretty good writing (I'm guessing Mortensen's V.O. is verbatim from the novel as that has some of the best lines.)
I think whether one likes or dislikes the film probably rests on what they think of Viggo Mortensen's performance, which I liked but I could understand others disliking. The boy is also good.