Very realisitic
So... I can't tell you whether or not this really simulates the experience of being face blind, but what I can tell you is that they did a good enough job that I, an actual person with prosopagnosia, couldn't tell that they were changing actors and stuff all the time. I had to ask a friend to tell me whether or not they were changing actors. I kinda figured they were because of the dramatic camera shots and music, but to me, nothing looked odd or out of place.
I think it was also pretty realistic to have the actress unable to figure out how to recognize people by body structure, movement, clothing, etc. I've had prosopagnosia since I was a child, so I've had a lifetime of developing the skills to quickly identify a person by these traits. But for a person who just got it, I'd be surprised if they were able to quickly figure out how to recognize people. Honestly, if there was anything unrealistic, it would be that. I'd be surprised that she was able to figure out how to recognize people by clothes/voice/etc. that fast.
Think of it this way. When you've been using one method your entire life to recognize people, you're not going to easily or quickly switch over to a new system. It takes a lot of time and practice to be able to do it effectively.
That said, I think the movie was really well done in terms of portraying what it is like to have prosopagnosia. After some years with it, I'm sure she wouldn't be nearly so awkward with her condition, and she would be able to compensate for it much better than she was during the film. Most people with prosopagnosia, like myself, get so good at compensating for it that we don't even realize that we've got it until much later in life, when we finally realize that other people have this seemingly magical ability to recognize faces.