I think it's annoying when celebrities think that whatever thought they have on politics is somehow gospel since they're famous. But sometimes I wonder - do the opinions matter more when the person expressing them is famous and more people will hear them? Anyway, here are a few articles on celebrities in the new right now for their activism or personal views where I think they seem sincere and will do some good.
Brad Pitt on Marriage Equity
Oprah's Angel Network
One Campaign - started by Bono
I do think that sometimes issues will get swept under the rug unless really pretty people talk about them. At the same time, I get annoyed when some celebrities who obviously don't know much about the topic try to sound involved.
I also know that a celebrity getting a horrible illness helps raise awareness (Michael J Fox and Parkinson's; Brooke Shields and PPD). And if that causes people to raise more money for research, that's a positive.