I agree with CharlotteWorks, it really is useless as in generating traffic and buzz unless it is news worthy.
Case studies, product releases would be some good reasons to use PR, if those categories are attractive.
But I do believe it is possible that PR sites can help with link building, but might just be an expensive link. I've looked at places like PRweb(have not used any PR places yet), and they offer PR and submission to NYtimes and AP, for about like $400.
Now I am not sure if these links from NYtimes and Associated Press are actual 100% guaranteed but if they are, it might be worth it just in a link building perspective.