I read this because I heard it had been banned at some schools, and nothing piques my interest more. I'm betting that, just as much as the trauma the protagonist suffers, the critical attitude toward school and school authorities--the principal, guidance counselor, several teachers--both in terms of ignoring her or bungling attempts at trying to get her to do better, and in terms of stupid rules and teaching methods and content, may have been responsible for the book's being banned in places. (My absolute favorite--and I'm saying this as an English teacher--is when one of the students, responding to the teacher's overemphasis on symbolism in Hawthorne, says unless Hawthorne left a book entitled "Symbolism in My Books," she could just be making it all up.) In this way, it's akin to The Chocolate War (though in the end, with the art teacher's help and the protagonist's breakthrough, Speak is indeed much more hopeful).
I'm sure other reviews have noted the astonishing way that this book manages to be so funny in the narrator's caustic way of observing school and school society. The author mentions in a Q&A in the back how her being a forced outsider allows for that. There are some great lines and images throughout, and the structure and pacing are well-done.
I'm not quite sure how I feel about one particular confrontation at the end, but I do like the way it's resolved and how that reinforces other places where girls helping girls is important. I'm a great admirer of those who can capture teenage voices, and this does a great job, and with important subject matter.