TheBoost said:

Can we think of any artist in any media that had a long and prolific career that didn't have some ventures into mediocrity?

I Martin Scorsese is a great director who is still putting out good films (Shutter Island, Departed, Aviator). I'm not a huge fan of departed, but most people seemed to like it. The only complaint about Scorsese is that Casino and Good Fellas are pretty much the same movie.

Leo DiCaprio is a case of the exact opposite. As a young actor I couldn't stand him, but now I find myself being big fan of his work (Gangs of New York, The Beach, etc). I guess he just needed to be introduced to Scorsese in order to be a respectable actor.

EDIT: I guess I have to take back what I said about DiCaprio's early career. He did a great job in What's Eating Gilbert Grape. "...going back up there, Gilbert"