Different publications vary greatly in their reliance on PR firms. At the bottom of the heap are the trade press, who make most of their money from advertising and would give the magazines away for free if advertisers would let them.  The average trade publication is a bunch of ads, glued together by just enough articles to make it look like a magazine. They're so desperate for "content" that some will print your press releases almost verbatim, if you take the trouble to write them to read like articles.
I encourage anyone to read Colin Clark’s articles on the debate over Iraq that ran in Defense News during the summer of 2002. Unlike The New York Times or The Washington Post, Clark’s reporting holds up very well.
Or read Michael Hardy’s coverage of the voting machines controversy in Federal Computer Week. Vastly superior to anything I have seen in any mainstream publication.
I have heard of trade publications giving away editorial for advertising, but never the kind of payola journalism that currently infests allegedly credible news organizations.