Just in case any description is spoilerific though, I'll put this behind a cut (on LJ anyway).
Definitely a tough movie. Very very well performed. I'd never seen Rosemarie DeWitt in anything. She reminds me a little of Jennifer Garner, but with a little better timing. Very believable story and acting. The music is really good - sort of dischordant violin that matches the feeling, or maybe creates the feeling, throughout so much of the movie. The story really is about Kym, who's played by Anne Hathaway. She does an amazing job playing the emotional turmoil. I did want her character to be a little more defiant, but I think that would have drawn the story out too much. She's not very believable as a smoker, but that's a pretty minor transgression.