I had ECT, a course of 25 shocks over a nine-month period. It was done against my will. It didn't help, but because it was non-consensual I didn't have much choice in the matter.
For me, the memory loss kicked in after about the tenth shock. I woke up from the anaesthesia and all my memories of the four-five previous years were completely gone. There isn't much research into memory loss and ECT (most docs will deny it causes any) so I don't know if it's a case of 'one shock gone wrong' or some culminative effect.