I agree to an extent. I have used PRweb on more than one site, and have had arguably good success considering it is one link from one site. PRweb is a 'trusted' authority in press releases as Ezinearticles.com is a trusted authority relating to articles.
The problem is that a PRweb submission is expensive. So is it worth it to you to spend one lump sum on a decent backlink, or spend that same money somewhere else. The link is permanent, so I guess that can be factored in.