The most poignant memory for me was during a short film shown at the beginning of the tour, when they said that at 3 hours (the expected tour length) we would spend more time there than the average Jewish person between arriving and being murdered.
The Hungarian Jews who arrived at Birkenau were dead within an hour or two. Heartbreaking. The room with the kids' voices playing and the drawings on the wall is particularly haunting. To anyone thinking of going, don't bother with the tours - they will rush you through both camps in a couple of hours. Auschwitz (the main camp) is an all day visit really, and then Birkenau can be done the following day. I'd recommend the Hotel Olecki, which is spotlessly clean, friendly, brilliantly located and does superb dumplings.
Apart from the actual monumental, Everest-sized mountain of evidence available (including the memoirs of the commandant of the camp itself), try and watch the film Denial, starring Rachel Weiss.
