Press releases used to have more impact on SEO than they do now, and the impact may lessen even more as time goes on. However, press releases still have value. I just released a PR over the last two days for a site of mine, and the response has been great. Lots of pickups, lots of visitors. That's what it's all about.
If I get some good, viable backlinks out of the process, that's a bonus. But what I really want are visitors who are interested in my topic. I need those people much more than I need backlinks.
But press releases have been getting a bum rap lately, and that's largely due to the amount of spam that has infiltrated the medium. Ah, spam...it always rears its ugly head. I have some topics on Google news that I am interested in, and every day, those areas see about 3/4 spam press releases. It's annoying to sift through them.
More importantly, those press releases have so little value. Why waste time and/or money on it? Unless your goal is to piss off your users, put out quality press releases. Make them NEWSWORTHY, for goodness sake. If there's nothing newsworthy about your site, don't put out a press release! And if there's nothing newsworthy about your site, then create something newsworthy! Be different. Stand out from the crowd. Do something new. Then put out a press release about it.
You'll be glad you did.