I use press releases once a month. Its good in terms of giving you traffic. When you optimize it for the keywords it can rank even on the first page. Over time if your page loses its ranking, these PR's still rank on the top.
I've only used free PR sites like prlog, widepr, pitchengine, to name a few and they've done their job so far.
I think its only with paid PR's that you get a link back.