I almost didn't watch the movie because of all the bad reviews, but I'm glad I did. I thought the movie was excellent. If you want an adaptation that is very faithful to the book, then you're probably going to be disappointed. Beatty and Clarisse's characters are very different from the book and a lot of things are changed or left out. It's easier for me to appreciate the movie because I'm not that fond of the book. There are some great things about Fahrenheit, but I think it is terribly plotted and a lot of it is just plain boring. I do like the ideas behind the book, though, and I loved the way the movie portrayed the importance of books and ideas, of thinking for yourself, and of standing up against tyranny. There were lots of places where they worked actual dialogue and ideas from the novel into the movie, which was awesome.
So if this is your favorite book of all time, you probably won't like that so much of it has been changed. And if you just want an action-packed sci-fi movie, you'll probaby also be bored by the philosophical discussions. But for everyone else, I think it's a well-made adaptation that is faithful to themes and ideas behind Bradbury's classic.