I visited Auschwitz/Birkenau a couple of years ago and would highly recommend it. It is really hard to put the experience into words.
I think whilst I was there I was somewhat numbed with shock, despite having read much about concentration camps and the holocaust. Nothing can prepare you for entire rooms full of the belongings stolen from the victims (tonnes of childrens clothes, shoes, luggage daubed with the owners details, glasses, hair...), it is these personal items that bring home the individuallity of each victim and the full horror of what happened to them.
Visiting Auschwitz provides the sort of education that can not be taught in a classroom or book.
Hiya
Also been there, and a few others in Poland besides.
It's as Malaika says - its a hell of an experience. I suggest you go either at the start or end of your stay in Poland - probably better at the start, as then anything else you do during your stay makes you feel glad those Nazi bastards didn't win. Also, be aware that Auschwitz (or Oswiecim as the Poles call it) is huge and there are a number of sites, though just visiting the main one is enough for most people
Enjoy it or endure it, but don't miss it out of your itinerary
Skotch