I was surprised to like this movie as much as I did because I find Diablo Cody's writing to be incredibly pat and annoying. But it was mostly toned down in this script, which was pretty dark and unglamorous.
Charlize Theron did a good job portraying an unhappy woman trying to reclaim her high school glory and Patton Oswalt gives a great supporting performance as one of her high school classmates. The scene where he describes what happened to him in high school vs. her description of what happened to him was just brilliant.
The movie had its problems and it is pretty dark, but overall I enjoyed it. I'd give it a 'B' (and Patton Oswalt an 'A').