Google has been gradually clamping down on guest blogging, at one time it was a great way of bring focused visitors to your site and at the same time picking up a few back links (links are the online equivalent of votes). But those days are long over. Google now detects if a blog is linking out to too many sites that themselves have a lot of blog backlinks.
So the message is avoid paid guest posting. Its quiet a different matter if blog owners out of their own initiative post reviews or links to you site. Its just those that do it at as a source of income, paid spam blogs or splogs that Google doesn’t like.
In the following videos above and below Matt Cutts head of Google’s web spam team explains more.
Over the years of working with Google I’ve noticed the trend myself a real world niche such as Directories, Forums and now blogging gets abused and abused by spammers until Google takes or has to take action to clean it up.