Saw it cold (hadn't read the book) and found it fascinating
I just came from a preview showing, and was impressed enough to react. I won't spoil it, but I can't vouch for what others may post.
I glanced at the overview of the reviews so far. As I write this, there are only 8 reviews, none from major outlets, lukewarm at best, with an average rating of 47. In a couple weeks (maybe by the time you see this), no doubt more reviews will be added; I wonder if any will be more positive.
http://www.metacritic.com/movie/american-pastoral/critic-reviews
It opens with the revelation that the major character has died, and then it goes back to the beginning of his story. That's all I'll say about the plot.
What I found compelling was that the traumas and emotional stumbling blocks that come up are on the messy side, by which I mean not the sort of thing that lends itself to a simple explanation. We had a sense of why a character might follow such-and-such a path, but we're really not sure; we have an idea of how another character might handle the resulting situation, but we can't really say he could have fixed it had he acted differently. I could believe in the dramatic dilemmas; they rang very true, so I became emotionally invested in the outcome, and I agonized along with the players.
The final resolution...well, I'll just say I was personally satisfied that I took something away from the viewing, but I couldn't elaborate without spoiling the show. I will say that the ending prompted modest applause (it was an industry screening), but I also heard people walking out saying how awful it was...which the person whom I was escorting also thought. So lots of people won't like this, and I'm not urging anyone to see it; I just thought it deserved a positive word.