This is a very well made and touching movie. I connected with Grace greatly, and even by the end I still felt sick and depressed just as she must have felt, rather than disliking her for what she did and being glad about the outcome (not to say she wasn't wrong, but still). That means the acting was very good and believable...except for the fact that Grace (and her mom or really anyone else, actually) did not shed any tears at any point, which is very unrealistic. Guess that was too hard for the actors...but it's really not such a big deal. Just saying.
While I thus was, initially, very disappointed that the ending did not turn out to be, as I put it, "for the best" or what "should have happened", I realize that that is actually a fact of life and this movie shows harsh reality about what mistakes and carelessness lead to. Clearly you can see that Grace normally wasn't such an irresponsible or risky person, but she made a mistake and indeed paid the consequences. However there are two things I would like to point out in light of some other reviews, based on my opinion (note that the following is particularly where MAJOR spoilers are included, by the way, so only read if you've seen the movie!).
First, I'm pretty sure that the "sadness" conveyed in the end is not so much about Grace having to go to prison for the crime she only just realized she committed, but that she caused her friend's death. Note that what actually caused the crash wasn't so much that Grace was too drunk to drive properly (although obviously that didn't help and could have caused a crash regardless), but that she turned around to yell at Jen in a way she really didn't mean and regretted. Think about it. In theory, that accident could have happened to a sober person, who was extremely stressed and distracted, in just the same way. Grace already cried after the confrontation with Pete because she realized she caused the crash. Her later sadness by the lake, which carried through and set the tone of the end the movie, was due loosing Jen, particularly with the last interaction they had in the car.
The other thing is that Jen's appearance is not really as a ghost. True, at times I would like to believe that, since it does make a lot more sense at times and is easier to understand. However, if that was true then Jen would have either expressed from the start that it was Grace's fault....and Jen would have been still upset, or at least hurt, that Grace was "cutting ties" between them right before the crash. Yet Jen was acting as if they were still friends...and they were, since Grace didn't really mean what she said in the car and did not hold on to those feelings afterward (in fact she regretted it so much she shut it out of her memory). Furthermore Jen even says to Grace that "I am a manifestation of your guilt". Of course, Jen does act more as a ghost than a hallucination by appearing as more of her own person than something out of Grace's mind. However, it just doesn't match up- the "Jen" that follows Grace around during the search for what happened does not know any more than Grace does, other than the fact that she sees and remembers some of Grace's memories better than Grace. That means that she is like Grace's conscience, in that sense.
Anyway, although the movie depressed me and has stuck to my mind for who knows how long, I think it has a good and well made plot, with very valuable lessons, ideas, and messages.