I read the book a long time ago and this is my first time watching the movie. It sucks. The book probably sucks too, it's been a while since I read it, and I was like 14 years old so I still thought Extended Universe books were excellent literature. I really wish I had seen this movie when I was 14, but back then you would have to find a physical copy of a movie if you wanted to watch it, not easy when that film is from Japan and $35 for a single DVD or VHS does not exist in your teen wallet.
[spoiler] - Anime-style gun battles: people get shot dozens of times and suffer no ill effects, except the one bullet delivered after a "cool" or "gut-punching" line of dialogue.
Due to the sheer amount of crushes there is hyper-inflation in the value of romance in this movie. In other words, if everyone is love with everyone else then there is actually zero meaningful romance in the movie.
Interesting characters die quickly while boring characters survive. Interesting characters never fight against interesting characters, its always Boring vs Interesting. Imagine if Sickle Girl had to confront the heroes, or if Bleach Haired Guy had any characterization other than deadly guy who teleports around the island. You never think to yourself, "Oh no, two characters I'm rooting for are fighting each other!"
Just pick two or three relationships with backstories! There's no point in squeezing an entire backstory between two characters in 90 seconds before they kill each other! Forget half of these stories! Take Track Suit Girl, build up the story -- ending with her stabbing her attempted rapist in the dick -- and make it bigger!
The acting is awful. Every time a character does their death soliloquy and pantomimes dying it was laughable, especially Beat Takeshi! Who is supposed to be good!
The use of classical music is the definition of pretentious: it shoots for deep meaning and falls flat on its stupid face. If its supposed to be a humourous juxtaposition then - no, this movie totally lacks humour and is entirely self-serious.
There is no meaning in the movie. A statement on older generations abusing and destroying the futures of their children, this movie is not. This movie is about screaming and arterial spray. (In the book, Japan is a dystopian country ruled by a totalitarian government. The winner of the Battle Royale receives an income and an apartment for life. These two details give the work enough meaning to push it past the threshold from "exploitation film" to "John Carpenter-level commentary")
As an exploitation film there are no memorable heroes or villains, just a couple of cool deaths, no stand out special FX, a lame-duck twist (Beat Takeshi was stabbed in the butt and that's when he decided to become... a lame dude...). IIRC the book's climatic battle is with characters lobbing grenades at each other while in a car chase. Probably outside this movie's budget :( [/spoiler]