A new study published in the British Medical Journal suggests perhaps they are not. From the New York Times:
[The study] found that the death rates from breast cancer and from all causes were the same in women who got mammograms and those who did not. And the screening had harms: One in five cancers found with mammography and treated was not a threat to the woman’s health and did not need treatment such as chemotherapy, surgery or radiation.
…
Researchers sought to determine whether there was any advantage to finding breast cancers when they were too small to feel. The answer is no, the researchers report.
Many of the cancers found by mammography posed no threat to women’s health and led to costly interventions that may not have been necessary. The study is not expected to change mammography guidelines though.