Yes, homeowners have been successful in getting their property taxes reduced. I've seen home values reduced by 20%. Depends what's going on in your neighborhood. Here's a link to the info on sfgov - http://www.sfbos.org/index.aspx?page=3696#.
Peter Fatooh, in my office is an agent who has served on the tax appeals board and works with people to fight incorrect assessments.We have another agent who currently serves on the board and so I can answer your question with some veracity. Some are successful and some aren't.