Surgery for Mental Illness

NY Times article entitled "Brain Surgery for Mental Ills Offers Both Hope and Risk." The article discusses an experimental surgery for severe mental illness, such as OCD. The surgery is risky, and not all patients improve, yet those that do report significant improvement. How might psychiatrists, neurologists and psychologists determine which patients are appropriate for surgery versus medication versus psychotherapy? What are the ethical implications of doing a surgery that seems to help, but no one knows why?