I was there in 1957 (as a 10yr old kid), and from what I understand there was more of it open then than now. The museum was phenomenal with the castings of the victims.
Visit it now and you'll really be impressed!
They've done a lot of work and you can see a lot more and it's better explained. One hint - rent the recorded audioguide or go with one of the real human person guides who hang out in front of the gates. They are about the same price.
I saw it a few years ago and they had more explanatory material on multi-lingual signs in front of the sites. Now, they have much less material and it's only in Italian (which I read, but maybe everybody else does not...).