I've not yet read the book, but I think the movie is spectacular and I like the manga as well (That and Bizenghast are the only two mangas I collect.).
However, I do find the plot to be somewhat unbelievable. Whilst 'government' might well see something similar as a good idea, the instinct of a mother to protect her children is too strong for a government which tried this to survive easily. If it was an entirely secret program I could understand, but in the movie at least it's a national TV event; not something parents would like too much.
Moreover, I simply don't see what anyone would believe is achieved by it. If it were a punishment then yes, or even if you could be exempted by good behaviour or academic success, but to just choose random classes of students and set them upon each other doesn't seem to serve any purpose for society. I know I certainly wouldn't feel any desire to feign respect if I knew it wasn't going to save me. This might be better explain/elaborated upon in the book though.
One problem I do have is that Kiriyama seems to exist only to reduce the numbers of the children. My friend who has read the book has told me that although his nature is elaborated upon more than in the manga (Which does actually go into his past, unlike the movie.), he still seems to serve that purpose. Disappointing.
