Used them a few years ago, at whatever level provided links with anchor text. We tried anchor text that the clients had never pushed before, so the boost could be identified, and these appeared effective, but ultimately faded over time. These were niche market areas, and we found it more productive on future efforts simply to identify the sites (blogs in all cases) that would pick up the story, and then offer customized versions of the article.
I've never managed to get enough coordination with a large clients' marketing departments to affect their press releases. By the time very large companies get something through branding and legal, they've rewritten the release to remove all effective keywords.
