Through-out the film it's clear that the ex-wife holds a grudge towards Emilia and she does act out on it a lot, delivering hateful comments and says things just with the intention to cause pain. But at the back of my mind this behavior was always justified. She had been wronged and betrayed, and I don't believe the creators were trying to make her out to be vindictive or spiteful - just hurt and angry because of that hurt. And Emilia was never given scenes where she tried to justify what she'd done, or where the viewers were supposed to choose her "side" in the whole ordeal.
The main theme of the movie was the child that Jack and Emilia lost and how that affected their relationship and most importantly how Emilia lived with the guilt of thinking she was the reason why their child had died. Jack points this out, saying that she hurts the people closest to her the most, and it affects their relationship so strongly that it comes to an end. The strongest scene for me, and the one scene that definitely showed what kind of person the ex-wife really was and where it became crystal clear that it wasn't about revenge or hatred but pain and betrayal, was when Emilia gets called to Carolyn's office so that the latter could make it clear that it wasn't Emilia's fault that her daughter died. Carolyn did this for her son, yes, but she didn't have to. If she'd been a horrible person, that a lot of viewers seem to think the director wanted her to look like, then she wouldn't have done it. This scene really implements that it was never about making anyone the victim or anyone the bad guy, but just about showing the situation for what it was and how everyone handled it differently, reacting as real human beings.
I saw no glorification or romanticism of the affair. I saw no trying to blame it all on the horrible ex-wife. I saw three people who ended up in each others life because of a mistake and they all handled it as best as they could.
As for the performances I thought Natalie Portman, Lisa Kudrow and Scott Cohen all did an amazing job. The movie itself is a very emotional journey if you're open to seeing the bigger picture instead of trying to find someone to blame or hate.