Matt Cutts on guest blogging:
Ultimately, this is why we can’t have nice things in the SEO space: a trend starts out as authentic. Then more and more people pile on until only the barest trace of legitimate behavior remains. We’ve reached the point in the downward spiral where people are hawking “guest post outsourcing” and writing articles about “how to automate guest blogging.”
So stick a fork in it: guest blogging is done; it’s just gotten too spammy. In general I wouldn’t recommend accepting a guest blog post unless you are willing to vouch for someone personally or know them well. Likewise, I wouldn’t recommend relying on guest posting, guest blogging sites, or guest blogging SEO as a linkbuilding strategy.
There are a handful of good guest blog posts a year — but only a handful. The vast majority of them are either entirely too self-serving or worse, spam.