You shouldn't put the SPE and Milgram experiments in the same basket. His experiments involved hundreds of participants from different demographics (mostly males though) and the conclusions he drew were backed up by his data. The setup he used is today deemed by most as unethical, but similar studies have reached the same conclusions.
A little known fact is that Milgram found, when the studied persons were free to set the voltage to administer, none of them increased it. His conclusion were that most people are not sadists and do not enjoy hurting others.
I have the book Obedience from Milgram at home, although I just skimmed through it. I think the situation is similar, Zimbardo would probably do more experiments as well weren't the first result so horrific.
What I am a bit worried of is that we are not able to replicate (because of the ethical problems), in one way or another, results of these studies. How can future generations become educated about these problems? Especially if we cast doubt over their results. Are our descendants doomed to figure out these things the hard way?
A little known fact is that Milgram found, when the studied persons were free to set the voltage to administer, none of them increased it. His conclusion were that most people are not sadists and do not enjoy hurting others.
Edit: Maybe I spoke to soon, see: http://www.psmag.com/navigation/health-and-behavior/electric... But the experiments where still very different in the number of test subjects and how scientifically they where conducted.