The disappointing part is the build up towards the end. I'm also not quite sure who the blame falls to, but the scene where the lead cries in front of the tree, doesn't provide the despair that would have created the tension that the finale needs.
What it very confrontingly succeeds in is the rediculous premisis that being drunk is an invitation for sex. And I can only applaud the movie for that, but the story telling...is lacking. In hindsight, I felt the bond between the two childhood friends didn't come to life, because of the absence of Nina and this also didn't help the finale build up. Some editing goes from a crying Cassy to a fully in control one. No plan forming. No rage. And perhaps not nutty enough.
Its flaws are not enough to pass this one by though, because it is unique where it succeeds.
This is something you should show in high schools and probably throughout college, instead of abstenance or condoms on bananas.