Went last October while on a 4 day break in Krakow, and was a surreal experience. Still remember it all in great detail, and probably always will, ad some of the displays/exhibitions really helped you to understand the true scale of the place. Thought Birkenhau was well worth the short bus trip as well.
Somewhere I think everyone should try and visit, obviously not a pleasant experience, but one that's well worth the effort of going. Oh, and Krakow's a cracking city to stay while you're out there of course.
Krakow is right up there among my favourite places. Absolutely love it! Did you go to the salt mine? Fan-bluddy-tastic!