The benefits of a site like prweb.com are the exposures (brand) that you'll get for the release. You may rank for up to a week for your phrases in the news verticals of the engines, but then that will fall away over time. Their worth is in getting a quality BL from a site that Google trusts - it's practically a paid link that Google endorses. So you need to decide if the money is worth it. Just as an FYI, I make all my clients do at least 1 release from that site.