Wikipedia...
Metallica
Main article: Metallica
Bob Rock produced Metallica's multiplatinum self-titled album (known as The Black Album) in 1991 and subsequent followups Load (1996), ReLoad (1997), Garage Inc. (1998) and S&M (1999). Metallica once again tapped Rock to produce their 2003 album St. Anger.
Rock played bass guitar for the recording of St. Anger.
A petition that over 20,000 fans signed was posted online. The petition called for Metallica to dump Rock as producer, claiming he had too much influence on the band's sound and musical format. Rock claimed the petition was hurtful for his children. He was also quoted as saying "Sometimes, even with a great coach, a team keeps losing. You have to get new blood in there." It is often said that Rock "Ruined the greatest metal band in history."
In February, 2006, Metallica chose producer Rick Rubin to produce their next album, ending the group's long-time relationship with Rock.
...Personally, I wouldn't go so far as to say he "ruined" Metallica. They were millionaires by the time he came along. I'm sure they all wanted to do something new. I couldn't expect them to keep that same anger and energy of a metal band, you know, after they were able to afford yachts?