There are people who were fairly well-known at the time they were in who left, though I wouldn't call them big stars. Diana Canova (who was on the TV series "Soap") left. So did Peggy Lipton of "Mod Squad" and her brother, Robert.
Christopher Reeve was in briefly and left, though I am pretty sure it was before he became famous. He talked a bit about his experiences in a book a few years back. He was cautious and diplomatic in what he said (along the lines of "It helps some people..."), but he made it clear he had no use for it. I don't think he was involved long enough to encounter any horror stories.