It is highly effective in bringing very manic patients back down to their normal baseline. The old horror scenarios of ECT made famous by One Flew Over the Cuckoos Nest just are not how it is any more. An individual is anesthetized, given a muscle paralyzing agent so there is virtually no movement, and ECT started. Most folks have a seizure for 30-45 seconds, wake up a few minutes later, and are just a bit groggy from the seizure and anesthetic agent. Manics come down a bit almost immediately.
Most depressives feel better after the first treatment, too.
This treatment is empirical, and it was just found patients do better with 2 treatments after they are better. Some folks can get 15 to 20 treatments, and some only 2 or 3 total. It is whatever works. The problems with ECT are many-fold, but manageable. First, it is really expensive, so insurance companies need a good rational for using it as an acute treatment.