Some press release sites are good, some are a waste of time.

PR Web is ok. We use them for a number of reasons

* Comprehensive release methods which really do get our releases picked up by other sites.
* Genuine editorial process. I've had to go back and edit releases sometimes in order to meet their standards before they would release them. This can only aid their credibility. Some PR sites will churn out any old carp.
* With an adequate payment level you can use keyword anchor text links. These do often seem to persist into the versions that are reproduced on other sites, so I think it's worth doing.
* You can attach images. News with a good image, in my experience, gets taken up by more outlets.

Be aware, though: if your press release is a spammy piece of crud just thrown out there in the hope of getting a couple of SEO links, then you have no idea how to use this tool. Find some genuine news and then do a release rather than writing pointless carp for the sake of putting a release out there.