I liked this. Original, intense, dark. Unlike anything I've watched before, really. Some of the best acting I've seen in a while, especially considering there is very little dialogue. In the end I couldn't help but think "If only they'd known all along..." Sounds about right for a sequel.
I wonder what might have happened if, instead of terminating the experiment after six days, Zimbardo decided to flip the script and have the "prisoners" act as "guards" and vice versa for the remainder of the time. So many theories and questions. Outstanding movie.