It very much depends on what your core objectives are as to whether using an online newswire is effective and also what niche your business operates within.

The success of using the likes of PRWeb may differ on a case by case basis.

Some tips to determine the success of a Press Release:
- Review some of the latest Press Releases within the most related category of the particular online newswire site you are looking to use e.g. http://www.prweb.com/newsbycategory/business/
- Search online for related keywords within the headline to see if they appear in Google News results or within the main Google search results for the company brand name
- Check where the release is visible online by searching for the headline in inverted commas
in the search engines e.g. https://www.google.com/search?q=%22Vollrath+Company+Showcases+Latest+Smallwares+and+Equipment+Innovations+at+National+Restaurant+Association+Show%22&num=100&espv=2&rlz=1CAHPZT_enGB672GB672&filter=0&biw=1366&bih=631&gws_rd=cr&ei=udQ_V7WXFce90gTwwKDwDQ
- Analyse how many times a press release is shared via social media by using the likes of www.buzzsumo.com or the Impatana Content Marketing Toolbar (available via https://chrome.google.com/webstore/detail/impactana-content-marketi/gidhppoakegddkkpmgoejeadomghnkie?utm_source=chrome-app-launcher). Many online press release websites offer "social media booster" packages for extra cost but are not likely to be as effective as if you promoted the release yourself via social media and your influencer network.

From a pure PR perspective, I'd recommending reading The PR Masterclass (https://play.google.com/store/books/details/Alex_Singleton_The_PR_Masterclass?id=2UJOAgAAQBAJ) which quotes....
"Although I don't recommend pay-for newswire services, there are proper newswires, funded by media organisations, which are worth contacting. Indeed, newspapers and broadcasters rely heavily upon them to find and filter news. In the UK and Ireland, general news is covered by the Press Association. The equivalent in Australia and New Zealand is the Australian Associated Press (AAP). In the United States and Canada, the Associated Press and The Canadian Press, respectively, take the lead. Reuters and Bloomberg are the main wire services about finance.

Each newswire service will provide you with details of how to submit your press release. They are, in my experience, extremely good at sorting through the material they receive by email."

For Search Engine Optimisation benefits using online press releases as a tactic have significantly diminished over the last 3 years so don't use online press releases if improving organic search visibility is one of your main goals.

My personal opinion is that online press release services are becoming a little outdated unless you have a particular niche targeting a local area that you may be able to gain PR traction in. Many online newswire services have pivoted to more social media listening and promotion tools over the last few years which shows how the impact of newswire releases has declined.

My suggestion would be to avoid these automated newswire services and instead:
- develop good direct PR relationships with relevant people in your relevant industry niche
- builds up your social influencer network by using tools such as Buzzsumo, Followerwonk, Buzzstream and http://klear.com/free-tools?source=whiteHeader and then promote your message via a blog or another type of engaging online content format
- Develop a targeted email list over time using software such as https://sumome.com/app/list-builder. CRM and email marketing are becoming more advanced every day and there are many success stories of how you can gain significant brand awareness through good email marketing.