We booked tickets through our hotel when we were in Krakow. I'm glad we did because when we arrived it was absolute chaos - there was a lengthy queue outside and people were bustling in/out of the doors and guides were trying to gather their groups. We were waiting outside for a good while still, even though we were on a guided group. Getting in was again a faff, they're just quite unorganised with tickets/cloakroom/security.
Once you're in, you're fine! Loads and loads of WW2 / Occupation of Poland info and detail. Not loads on Schindler, so bear that in mind. Our tour guide was 10/10 - I think her name was Martina, with Krakow Direct.