like with anything, you will get a boost of traffic for a bit, as the content slides down to other pages, then obviously your traffic will start to decline; Press Releases are good for diversity links. People use PR because of the flash traffic in the beginning could get them sales, then they just buy the PR over, and over again; Backlinks will obviously be no-follow, however, it is also possible that you could pick up side links that are do-follow, and this really depends quality of your site, quality of what you are selling, etc...etc...You never know, a big wig may read your news story, and decide to do his own news story on his site, thus giving you a backlink that way... I would advise that you do not use press release links just because or you think having 100s of no-follow links are cool. You should only use press releases when you already have a proven product, otherwise, you will end up very disappointed because you were trying out something you never tried before.... and to answer your question, if you use a legit press release provider, then you could end up getting 100s of no-follow backlinks, if this of any use to you.... think about it... If PR worked out for everyone... then everyone would be doing it, right?
they will make your product or brand go viral for few days with little sales if you have many products to sell on website.