For me, my first mammogram really was painful. It was like a Chinese burn, that got more painful the longer I was squashed and with each repeated squash. I was sore for several days after and had two sore, itchy patches that came back each month. Once I realised it was the beginnings of mastitis, I took some ibuprofen for a few days and it settled. Unfortunately, as this was my first time, I knew nothing different.
The second time couldn't have been more different. I didn't feel a thing, partly because of the painkillers and partly because the operator was careful, lined everything up properly took only the two shots each side, was quicker, and did not leave me hanging on tip-toes. Even after the follow-up, where I had multiple mammograms with different attachments, I was only mildly sore afterwards for a couple of days, if I poked it.