There are two distinctly different areas that accept press releases: media outlets and press release websites. Often, businesses submit to both to increase the visibility and searchability of the content, and is a good rule of thumb for marketing.
Just like everything else, there are some benefits and drawbacks for using these type of sites. One of the major disadvantages is that the press don't troll these sites for story ideas.
If you're expecting your story to be picked up by The New York Times because you submitted to a press release site, then you're out of luck. But, if you recognize that by submitting to these sites you're boosting your SEO, then you're totally in luck.
Figure out what are the top press release websites and spend some time looking through their submission requirements.
Most websites that accept press releases have standard submission guidelines including press release length, content, type of document (PDF or text) and attachments (photos and graphs).
Also, look at the most commonly submitted topics including the caliber of topics. Are people submitting what sounds like “legit” press releases?
Use a search engine and type in some of the keywords found in a handful of press releases that you find. See whether the press releases are found by the search engine and how high the rankings are.
If you can't find them, then remove that website from your list of candidates.
Submit a generic press release to each press release website so you get the feel about their process. Then, repeat the “search” process to see if your press releases come up. The ones you want to use will pop up in a Google search within a few hours (sometimes sooner).
If you are looking to use press release submission sites as a way to boost SEO value, SimplyCast has a press release marketing tool that will save you a lot of time. Basically you enter your information once and let the software auto-complete it for each submission site.
Happy news spreading.