I thought they did a really great job with this movie. I thought they stuck to the book pretty well, there were lots of lines straight from the book, and while some of the events happened out of order it didn't do anything to change the overall plot.
I thought that Kristen Stewart did a great job. When reading the book you really got the impression of how much they loved each other, but how she's still an awkward teenager, and that came through in the movie.
I heard a lot of complaining at the end of the movie, but I disagree. I think it was well done and very true to the book!